zotero-db/storage/JJN25NRM/.zotero-ft-cache

DUBLIN UNIVERSITY PRESS SERIES.
A HISTORY
OF THE
THEORIES OF AETHER AND ELECTRICITY
FKOM THE AGE OF DESCAKTES TO THE CLOSE OF THE NINETEENTH CENTURY.
BY
E. T. WH1TTAKER,
Hon. Sc.D. (DubL}; I.E.S.; Royat Astronomer of Ireland.
LONGMANS, GREEN, AND CO.,
39 PATERNOSTER ROW, LONDON, NEW YORK, BOMBAY, AND CALCUTTA.
HODGES, FIGGIS, & CO., LTD., DUBLIN.
1910.

MM*
DUBLIN : PRINTED AT UHE UNIVERSITY PRESS,
BY PONSONBY AND OIBRS.

THE author desires to record his gratitude to Mr. W. W.
EOUSE BALL, Fellow of Trinity College, Cambridge, and to Professor W. McF. ORR, F.R.S., of the Royal College of Science for Ireland ; these friends have read the proof-sheets, and have made many helpful suggestions and criticisms.
Thanks are also 'due to the BOARD OF TRINITY COLLEGE, DUBLIN, for the financial assistance which made possible the
publication of the work.
236360

CONTENTS.

CHAPTEK I.

y THE THEORY OF THE AETHER IN THE SEVENTEENTH CENTURY.

......1 Matter and aether, .

.

.

.

.

.

The physical writings of Descartes,

Page 2

........ Early history of magnetism : Petrus Peregrinus, Gilbert, Descartes,
Fermat attacks Descartes' theory of light : the principle of least

7

time,

10

Hooke's undulat>ry theory : the advance of wave -fronts, .

Newton overthrows Hooke's theory of colours,

.

.

.

11

.15

Conception of the aether in the writings of Newton,

.

. 17

Newton's theories of the periodicity of homogeneous light, and of

fits of easy transmission,

.

.

,20

The velocity of light : Galileo, Roemer,

.

.

.

.21

Huygens' Traite de la lumiere : his theories of the propagation of

waves, and of crystalline optics,

.

.

.22

Newton shows that rays obtained by double refraction have sides :

his objections to the undulatory theory,

.

.

.28

X

CHAPTER II.

ELECTRIC AND MAGNETIC SCIENCE, PRIOR TO THE INTRODUCTION OF THE POTENTIALS.

The electrical researches of Gilbert : the theory of emanations,

.

29

....... State of physical science in the first half of the eighteenth century,

Gray discovers electric conduction : Desaguliers, .

.

The electric fluid,

32 37 38

..... Du Fay distinguishes vitreous and resinous electricity,

Xollet's effluent and affluent streams,

.

.

The Leyden phial,

. .
.

.39 .40
. 41

The one-fluid theory : ideas of Watson and Franklin,

.

. 42

Final overthrow by Aepinus of the doctrine of effluvia,

.

Priestley discovers the law of electrostatic force, .

.

. 48
.50

viii

Contents.

Cavendish,

.

Michell discovers the law of magnetic force, .

The two-fluid theory : Coulomb,

.

.

Limited mobility of the magnetic fluids,

.

Poisson's mathematical theory of electrostatics,

... Page 51

.

.

.54

.

.

.56

.

.58

.

.

.59

The equivalent surface- and volume-distributions of magnetism :

Poisson's theory of magnetic induction,

.

.

.64

Green's Nottingham memoir,

.

.

.

.

.65

CHAPTER III.

GALVANISM, FROM GALVANI TO OHM.

... Sulzer's discovery,

.

....... Galvanic phenomena,

Rival hypotheses regarding the galvanic fluid,

,

.

....... The voltaic pile,

..... Nicholson and Carlisle decompose water voltaically,

.

.67

68
.70

72

.

75

Davy's chemical theory of the pile,

76

Grothuss' chain,

.

.

.

.

.

.

.78

De La Rive's hypothesis,

.

.

.

.

.

.79

Berzelius' scheme of electro-chemistry,

.

.

.

... Early attempts to discover a connexion between electricity

magnetism,

.

.80
and 83

Oersted's experiment : his explanation of it,

.

.

.85

The law of Biot and Savart, .

.

.

.

.

.86

The researches of Ampere on electrodynamics,

.

.

87

Seebeck's phenomenon,

.

.

.

.

.

.90

Davy's researches on conducting power,

.

.

.

.94

Ohm's theory : electroscopic force, .

.

.

.

.95

CHAPTER IV.

THE LUMINIFEBOUS MEDIUM, FROM BRADLEY TO FRESNEL.

.99 Bradley discovers aberration,

.

.

.

.

.... John Bernoulli's model of the aether,

100

Maupertuis and the principle of least action,

.

.

. 102

.... Views of Euler, Courtivron, Melvill,

104

... ... Young defends the undulatory theory, and explains the colours of

thin plates,

105

Laplace supplies a corpuscular theory of double refraction, .

. 109

Contents.

ix

Young proposes a dynamical theory of light in crystals,

.

.

....... Researches of Malus on polarization,

Recognition of biaxal crystals,

.

Fresnel successfully explains diffraction,

.

.

.

His theory of the relative motion of aether and matter,

.

.

Young suggests the transversality of the vibrations of light,

.

Fresnel discusses the dynamics of transverse vibrations, .

.

..... Fresnel's theory of the propagation of light in crystals,

.

.

Hamilton predicts conical refraction,

.

.

.

Fresnel's theory of reflexion,

Page
110 Ill 113 114 115 121 123 125 ] 31 133

CHAPTER V.

I
,THE

AETHER

AS

AN

ELASTIC

SOLID.

Astronomical objection to the elastic-solid theory : Stokes'

hypothesis.

.

.

.

.

.

.

.137

Navier and Cauchy discover the equation of vibration of an elastic

solid,

139

.......... Poisson distinguishes condensational and distortional waves,

.

Cauchy's first and second theories of light iq, crystals,

.

.

Cauchy's first theory of reflexion,

His second theory of reflexion,

141 143 145 147

The theory of reflexion of MacCullagh and Neumann,

.

. 148

Green discovers the correct conditions at the boundaries, .

. 151

Green's theory of reflexion : objections to it,

.

.

. 152

MacCullagh introduces a new type of elastic solid, .

.

. 154

W. Thomson's model of a rotationally-elastic body,

.

. 157

Cauchy's third theory of reflexion : the contractile aether, .

. 158

....... Later work of W. Thomson and others on the contractile aether, .

Green's first and second theories of light in crystals,

.

.

Influence of Green, Researches of Stokes on the relation of the direction of vibration of
.... light to its plane of polarization, .... The hypothesis of aeolotropic inertia,

159 161 167
168 171

Rotation of the plane of polarization of light by active bodies,

. 173

MacCullagh's theory of natural rotatory power,

.

. 175

MacCullagh's and Cauchy's theory of metallic reflexion,

.

.

Extension of the elastic -solid theory to metals,

.

.

.... Lord Rayleigh's objection,

.

..... Cauchy's theory of dispersion,

.

.

Boussinesq's elastic-solid theory,

177 179 181 182 185

x

Contents.

CHAPTEE VI.

FARADAY.

Page

Discovery of induced currents : lines of magnetic force,

.

Self-induction,

.

.

.

.

.

.

. 189
.193

Identity of frictional and voltaic electricity : Faraday's views on the

nature of electricity,

.

.

.

.

. 194

Electro-chemistry, .

.

"..

.

*.

.

.

. 197

Controversy between the adherents of the chemical and contact

hypotheses,

.

.

.

.

.

. 201

The properties of dielectrics, . Theory of dielectric polarization :

Mossotti,

.

.

:

.

Faraday,

.

.

.

.

W. Thomson,

.

.

. 206 and
.211

The connexion between magnetism and light,

.

.

. 213

Airy's theory of magnetic rotatory polarization,

.

. 214

Faraday's Thoughts on Ray -Vibrations,

.

..''-.

.

. 217

Researches of Faraday and Pliicker on diamagnetism,

.

. 218

CHAPTER VII.

THE MATHEMATICAL ELECTRICIANS OF THE MIDDLE OF THE NINETEENTH
CENTURY.

F. Neumann's theory of induced currents : the electrodynamic

potential,

.

.

.

W. Weber's theory of electrons,

.

Riemann's law,

.

.

.

.

.

;.

... .

.

.

. 222
.225
. 231

.... v-Proposals to modify the law of gravitation, .

..

.

.

Weber's theory of paramagnetism and diamagnetism : later theories,

Joule's law : energetics of the voltaic cell,

232 234 239

............ Researches of Helmholtz on electrostatic and electrodynamic energy,
W. Thomson distinguishes the circuital and irrotational magnetic
vectors,
His theory of magnecrystallic action,

242
244 245

His formula for the energy of a magnetic field,

.

.

. 247

Extension of this formula to the case of fields produced by currents, 249

Kirchhoff identifies Ohm's electroscopic force with electrostatic

potential,

.

.

.

.

.

.

/ 251

The discharge of a Leyden jar : W. Thomson's theory,

.

. 253

...... The velocity of electricity and the propagation of telegraphic signals,
Clausius' law of force between electric charges : crucial experiments, Nature of the current,

254 261 263

The thermo-electric researches of Peltier and W. Thomson,

264

Contents.

xi

CHAPTER VIII.

MAXWELL.

Gauss and Riemann on the propagation of electric actions, .
.... Analogies suggested by W. Thomson,
..... Maxwell's hydrodynamical analogy,
...... The vector potential,

Page . 268
269 271 273

...... Linear and rotatory interpretations of magnetism, .

.

.

Maxwell's mechanical model of the electromagnetic field, .

.

274 276

Electric displacement,

279

Similarity of electric vibrations to those of light, .

.

. 281

Connexion of refractive index and specific inductive capacity,

... Maxwell's memoir of 1864, .

.

. 283
.284

The propagation of electric disturbances in crystals and in metals, . 288

...... Anomalous dispersion,

291

The Max well -Sellmeier theory of dispersion,

.

.

.

Imperfections of the electromagnetic theory of light,

.

.

...... The theory of L. Lorenz,

Maxwell's theory of stress in the electric field,

.

.

.

...... The pressure of radiation,

292 295 297 300 303

Maxwell's theory of the magnetic rotation of light, .

.

. 307

CHAPTER IX.

MODELS OF THE AETHER.

Analogies in which a rotatory character is attributed to magnetism, 310

...... Models in which magnetic force is represented as a linear velocity,
Researches of W. Thomson, Bjerknes, and Leahy, on pulsating and
oscillating bodies,

311 316

MacCullagh's quasi-elastic solid as a model of the electric medium, 318

The Hall effect,

.

.

.

.

.

.320

Models of Riemann and Fitz Gerald,

.

.

.

. 324

Vortex-atoms,

.

.

.

.

.

.

.326

The vortex-sponge theory of the aether : researches of W. Thomson,

Fitz Gerald, and Hicks, ,

.

.

.

.

.327

CHAPTER X.

THE FOLLOWERS OF MAXWELL.

....... Helmholtz and H. A. Lorentz supply an electromagnetic theory of

reflexion,

337

Crucial experiments of Helmholtz and Schiller,

.

.

. 338

xii

Contents.

Page

Convection -currents : Rowland's experiments,

.

.

. 339

The moving charged sphere : researches of J. J. Thomson, Fitz Gerald,

and Heaviside, .

.

.

.

.

.

.

.... Conduction of rapidly -alternating currents,

........... Fitz Gerald devises the magnetic radiator,

340 344 345

Poynting's theorem,

347

Poynting and J. J. Thomson develop the theory of moving lines of

force,

.

.

.

.

.

.

. 349

Mechanical momentum in the electromagnetic field,

.

.

New derivation of Maxwell's equations by Hertz, .

.

.

.... Hertz's assumptions and Weber's theory,

.... Experiments of Hertz on electric waves,

The memoirs of Hertz and Heaviside on fields in which material

...... bodies are in motion,
..... The current of dielectric convection,

... Kerr's magneto-optic phenomenon, .
.... Rowland's theory of magneto-optics,

352 353 356 357
365 367 368 369

The rotation of the plane of polarization in naturally active bodies, 370

CHAPTER XI.

CONDUCTION IN SOLUTIONS AND GASES, FROM FARADAY TO

J. J. THOMSON.

The hypothesis of Williamson and Clausius,
...... Migration of the ions,
.... The researches of Hittorf and Kohlrausch,
.......... Polarization of electrodes,

.

.

.

Electrocapillarity,

.

Single differences of potential,

.

.

.

.

.

.... Helmholtz' theory of concentration-cells,

... ... Arrhenius' hypothesis,

... The researches of Nernst,

.

..... Earlier investigations of the discharge in rarefied gases,

.

.

Faraday observes the dark space, Researches of Pliicker, Hittorf, Goldstein, and Varley, on the

.... cathode rays,

.

.... Crookes and the fourth state of matter,

....... Objections and alternatives to the charged-particle theory of

cathode rays,

372 373 374 375 376 379 381 383 386 390 391
393 394
395

Giese's and Schuster's ionic theory of conduction in gases, .

. 397

J. J. Thomson measures the velocity of cathode rays,

.

. 400

Contents.

xiii

Discovery of X-rays : hypotheses regarding them, .

.

Further researches of J. J. Thomson on cathode rays : the ratio m/e,

Vitreous and resinous electricity,

.

.

.

Determination of the ionic charge by J. J. Thomson,

.

.

Becquerel's radiation : discovery of radio-active substances,

.

Page 401 404 406 407 408

CHAPTER XII.

THE THEORY OF AETHER AND ELECTRONS IN THE CLOSING YEARS OF THE NINETEENTH CENTURY.

Stokes' theory of aethereal motion near moving bodies,

.

. 411

...... Astronomical phenomena in which the velocity of light is involved,

Crucial experiments relating to the optics of moving bodies,

.

..... Lorentz' theory of electrons,

The current of dielectric convection : Rontgen's experiment,

.

The electronic theory of dispersion,

413 416 419 426 428

Deduction of Fresnel's formula from the theory of electrons,

. 430

Experimental verification of Lorentz' hypothesis, .

.

. 431

Fitz Gerald's explanation of Michelson's experiment,

.

. 432

Lorentz' treatise of 1895,

.

.

.

.

.

. 433 .

Expression of the potentials in terms of the electronic charges, Further experiments on the relative motion of earth and aether,

. 436 . 437

Extension of Lorentz' transformation : Larmor discovers its

connexion with Fitz Gerald's hypothesis of contraction,

. 440

Examination of the supposed primacy of the original variables :

fixity relative to the aether : the principle of relativity,

.

..... The phenomenon of Zeeman,

Connexion of Zeeman's effect with the magnetic rotation of light, .
..... The optical properties of metals,

............. The electronic theory of metals,

444 449 452 454 456

Thermionics,

464

INDEX,

.

470

MEMOKANDUM ON NOTATION.
VECTORS are denoted by letters in clarendon type, as E.
E E E E The three components of a vector are denoted by x , y , z ;
and the magnitude of the vector is denoted by E, so that

The vector product of two vectors E and H, which is denoted

H by [E . H], is the vector whose components are (Ey z - E^H^

E H E*H E H ZX

Z, EtHy - y x}. Its direction is at right angles to the

direction of E and H, and its magnitude is represented by twice the

area of the triangle formed by them.

H E H E E E^. The scalar product of E and

is

X X+

+
yy

It is

denoted by (E . H).

The quantity

OJ^j

(1 jjj y

O Jjj

-f

-I-

is denoted by div E.

The vector whose components are

f J

*

*

t

^ . y_

*\

is denoted by curl E.

V If denote a scalar quantity, the vector whose components are

^ - 8F
5T

* 8F
^7'

- 9F\
-5T

1S denoted b7 grad

898 The symbol V is used to denote the vector operator whose

components are

,

,

.

dx dy 82

Differentiation with respect to the time is frequently indicated by a dot placed over the symbol of the variable which is differentiated.

THEORIES OF AETHER AND ELECTRICITY.

CHAPTEK I.

THE THEORY OF THE AETHER IN THE SEVENTEENTH CENTURY.

THE observation of the heavens, which has been pursued continually from the earliest ages, revealed to the ancients the regularity of the planetary motions, and gave rise to the conception of a universal order. Modern research, building on this foundation, has shown how intimate is the connexion

between the different celestial bodies. They are formed of the

same kind of

matter ;

they are

similar

in

origin

and

history ;

and across the vast spaces which divide them they hold

perpetual intercourse.
Until the seventeenth century the only influence which was
known to be capable of passing from star to star was that of light. Newton added to this the force of gravity ; and it is now
recognized that the power of communicating across vacuous regions is possessed also by the electric and magnetic attractions.
It is thus erroneous to regard the heavenly bodies as isolated
in vacant space; around and between them is an incessant conveyance and transformation of energy. To the vehicle of this activity the name aetlier has been given.
The aether is the solitary tenant of the universe, save for that infinitesimal fraction of space which is occupied by ordinary matter. Hence arises a problem which has long engaged
attention, and is not yet completely solved : What relation subsists between the medium which fills the interstellar void

and the condensations of matter that are scattered throughout

it?

B

l'
$5

r The ^Theory of the -Aether

The history of this problem may be traced back continuously

to the earlier half of the seventeenth century. It first emerged

clearly in that reconstruction of ideas regarding the physical

universe which was effected by Eene Descartes.

Descartes was born in 1596, the son of Joachim Descartes,

Counsellor to the Parliament of Brittany. As a young man he

followed the profession of arms, and served in the campaigns of

Maurice of Nassau, and the Emperor ; but his twenty-fourth

year brought a profound mental crisis, apparently not unlike

those which have been

recorded of many religious

leaders ;

and

he resolved to devote himself thenceforward to the study of

philosophy.
The age which preceded the birth of Descartes, and that in which he lived, were marked by events which greatly altered the prevalent conceptions of the world. The discovery of America, the circumnavigation of the globe by Drake, the overthrow of the Ptolemaic system of astronomy, and the invention of the telescope, all helped to loosen the old foundations and to make plain the need for a new structure. It was this that Descartes set himself to erect. His aim was the most ambitious

that can be conceived ;

it was nothing less than to create from

the beginning a complete system of human knowledge.

Of such a system the basis must necessarily be metaphysical ;

and this part of Descartes' work is that by which he is most

widely known. But his efforts were also largely devoted to the

mechanical explanation of nature, which indeed he regarded as

one of the chief ends of Philosophy.*

The general character of his writings may be illustrated by

a comparison with those of his most celebrated contemporary, f

Bacon clearly defined the end to be sought for, and laid down

the method by which it was to be attained; then, recognizing

that to discover all the laws of nature is a task beyond the

* Of the works M'hich bear on our present subject, the Dioptrique and the Me'teores were published at Leyden in 1638, and the Principia Philosophiae at Amsterdam in 1644, six years before the death of its author.
t The principal philosophical works of Bacon were written about eighteen years
before those of Descartes.

in the SeventeentJi Century.

3

powers of one man or one generation, he left to posterity the
work of filling in the framework which he had designed. Descartes, on the other hand, desired to leave as little as possible for his successors to do ; his was a theory of the universe, worked

out as far as possible in every detail. It is, however, impossible

to derive such a theory inductively unless there are at hand

sufficient

observational

data

on

which

to

base

the

induction ;

and as such data were not available in the age of Descartes,

he was compelled to deduce phenomena from preconceived

principles and causes, after the fashion of the older philosophers.
To the inherent weakness of this method may be traced the

errors that at last brought his scheme to ruin. The contrast between the systems of Bacon and Descartes is
not unlike that between the Eoman republic and the empire of Alexander. In the one case we have a career of aggrandizement

pursued with patience

for

centuries ;

in

the

other

a

growth of

fungus-like rapidity, a speedy dissolution, and an immense

influence long exerted by the disunited fragments. The

grandeur of Descartes' plan, and the boldness of its execution,

stimulated scientific thought to a degree before unparalleled ;

and it was largely from its ruins that later philosophers

constructed those more valid theories which have endured to

our own time.

Descartes regarded the world as an immense machine,
operating by the motion and pressure of matter. " Give me
matter and motion," he cried, " and I will construct the universe."
A peculiarity which distinguished his system from that which
afterwards sprang from its decay was the rejection of all forms
of action at a distance he assumed that force cannot be com;

municated except by actual pressure or impact. By this
assumption he was compelled to provide an explicit mechanism in order to account for each of the known forces of nature a

task evidently much more difficult than that which lies before those who are willing to admit action at a distance as an
ultimate property of matter.
Since the sun interacts with the planets, in sending them B2

4

The Theory of the Aether

light and heat and influencing their motions, it followed from Descartes' principle that interplanetary space must be a plenum,,

occupied by matter imperceptible to the touch but capable of

serving as the vehicle of force and light. This conclusion in

turn determined the view which he adopted on the all- important

question of the nature of matter.

Matter, in the Cartesian philosophy, is characterized not by

impenetrability, or by any quality recognizable by the senses,,

but

simply

by

extension ;

extension constitutes

matter, and

matter constitutes space. The basis of all things is a primitive,,

elementary, unique type of matter, boundless in extent and

infinitely divisible. In the process of evolution of the universe

three distinct forms of this matter have originated, correspond-

ing respectively to the luminous matter of the sun, the

transparent matter of interplanetary space, and the dense, opaque matter of the earth. " The first is constituted by what

has been scraped off the other particles of matter when they

were

rounded ;

it

moves

with

so

much

velocity that when

it

meets other bodies the force of its agitation causes it to be

broken and divided by them into a heap of small particles that

are of such a figure as to fill exactly all the holes and small

interstices which they find around these bodies. The next type

includes most of

the rest of matter ;

its

particles are spherical,

and are very small compared with the bodies we see on the

earth ;

but

nevertheless

they have a

finite

magnitude, so

that

they can be divided into others yet smaller. There exists in

addition a third type exemplified by some kinds of matter

namely, those which, on account of their size and figure, cannot be so easily moved as the preceding. I will endeavour to show that

all the bodies of the visible world are composed of these three

forms of matter, as of three distinct elements ; in fact, that the sun

and the fixed stars are formed of the first of these elements, the

interplanetary spaces of the second, and the earth, with the

planets and comets, of the third. For, seeing that the sun and

the fixed stars emit light, the heavens transmit it, and the earth,
the planets, and the comets reflect it, it appears to me that there

in the Seventeenth Century.

5

is ground for using these three qualities of luminosity, transparence, and opacity, in order to distinguish the three elements

of the visible world.*

According to Descartes' theory, the sun is the centre of an immense vortex formed of the first or subtlest kind of inatter.f

The vehicle of light in interplanetary space is matter of the second kind or element, composed of a closely packed assemblage of globules whose size is intermediate between that of the vortex-matter and that of ponderable matter. The globules of the second element, and all the matter of the first element, are

constantly straining away from the centres around which they

turn, owing to the centrifugal force of the vortices ;J so that the

globules are pressed in contact with each other, and tend to

move outwards, although they do not actually so move. It is

the transmission of this pressure which constitutes light ; the

action of light therefore extends on all sides round the sun and

fixed

stars,

and

travels instantaneously to

any

distance. |j

In

the Dwptrique$ vision is compared to the perception of the
presence of objects which a blind man obtains by the use of his

stick ;

the

transmission

of

pressure along

the

stick

from

the

object to the hand being analogous to the transmission of

pressure from a luminous object to the eye by the second kind

of matter.

Descartes

supposed the

" diversities

of colour and light " to

he due to the different ways in which the matter moves.** In

the Meteores,^ the various colours are connected with different

rotatory velocities of the globules, the particles winch rotate most

rapidly giving the sensation of red, the slower ones of yellow, and

the slowest of green and blue the order of colours being taken

from the rainbow. The assertion of the dependence of colour

* Principia, Part iii, 52.

t It is curious to speculate on the impression which would have been produced had the spirality of nehulse heen discovered hefore the overthrow of the Cartesian

theory of vortices.

J Ibid., 55-59. ** Principia, Part iv,

Ibid., 195.

63.

Ibid., 64.
||

ft Discours Huitieme.

IT Discours premier.

6

The Theory of the Aether

on periodic time is a curious foreshadowing of one of the

great discoveries of Newton.

The general explanation of light on these principles was

amplified by a more particular discussion of reflexion and

refraction. The law of reflexion that the angles of incidence

and refraction are equal

had been known to the Greeks but ;

the law of refraction that the sines of the angles of incidence

and refraction are to each other in a ratio depending on the
media was now published for the first time.* Descartes gave

it as his own but he seems to have been under considerable ;

obligations to Willebrord Snell (b. 1591, d. 1626), Professor of
Mathematics at Leyden, who had discovered it experimentally
(though not in the form in which Descartes gave it) about 1621. Snell did not publish his result, but communicated it in

manuscript to several persons, and Huygens affirms that this

manuscript had been seen by Descartes.

Descartes presents the law as a deduction from theory.

This, however, he is able to do only by the aid of analogy ;.

when rays meet

ponderable bodies,

"
they

are

liable to be

deflected or stopped in the same way as the motion of a ball or

a stone

"
impinging 011 a body ;

for

" it is

easy to

believe that

the action or inclination to move, which I have said must be

taken for light, ought to follow in this the same laws as

motion."f Thus he replaces light, whose velocity of propagation

he believes to be always infinite, by a projectile whose velocity varies from one medium to another. The law of refraction is

then proved as followsJ :
A Let a ball thrown from meet at B a cloth CBE, so weak
that the ball is able to break through it and pass beyond, but with its resultant velocity reduced in some definite proportion,,
say 1 : k.
Then if BI be a length measured on the refracted ray
equal to AB, the projectile will take k times as long to
describe BI as it took to describe AB. But the component

* Dioptrique, Discount second.

t Jbid., Discows premier.

% Ibid., Discotirs second.

in the Seventeenth Century.

7

of velocity parallel to the cloth must be unaffected by the

BE impact; and therefore the projection

of the refracted ray

BC must be k times as long as the projection

of the incident

I

ray. So if i and r denote the angles of incidence and refraction,

we have

BE BC

or the sines of the angles of incidence and refraction are in a constant ratio this is the law of refraction.
;
Desiring to include all known phenomena in .his system, Descartes devoted some attention to a class of effects which
were at that time little thought of, but which were destined to play a great part in the subsequent development of Physics.
The ancients were acquainted with the curious properties possessed by two minerals, amber (riXtKrpov) and magnetic iron ore (77 \iOos Mayv?}r/e). The former, when rubbed, attracts light bodies : the latter has the power of attracting
iron.
The use of the magnet for the purpose of indicating direction at sea does not seem to have been derived from classical
antiquity ; but it was certainly known in the time of the Crusades. Indeed, magnetism was one of the few sciences which progressed during the Middle Ages ; for in the thirteenth
century Petrus Peregrinus,* a native of Maricourt in Picardy,
made a discovery of fundamental importance. Taking a natural magnet or lodestone, which had been
rounded into a globular form, he laid it on a needle, and marked
* His Epistola was written in 1269.

8

The Theory of the Aether

the line along which the needle set itself. Then laying the

needle on other parts of the stone, he obtained more lines in
the same way. When the entire surface of the stone had been

covered with such lines, their general disposition became evident;

they formed circles, which girdled the stone in exactly the same

way as meridians of longitude girdle the

earth ;

and there were

two points at opposite ends of the stone through which all the

circles passed, just as all the meridians pass through the Arctic

and Antarctic poles of the earth.* Struck by the analogy,

Peregrinus proposed to call these two points the poles of the

magnet : and he observed that the way in which magnets set

themselves and attract each other depends solely on the position

of their poles, as if these were the seat of the magnetic power.

Such was the origin of those theories of poles and polarization

which in later ages have played so great a part in Natural

Philosophy.
The observations of Peregrinus were greatly extended not long before the tune of Descartes by William Gilberd or Gilbertf (6. 1540, d. 1603). Gilbert was born at Colchester: after studying at Cambridge, he took up medical practice in London, and had the honour of being appointed physician to Queen Elizabeth. In 1600 he published a work* on Magnetism and Electricity, with which the modern history of both subjects

begins.
Of Gilbert's electrical researches we shall speak later : in magnetism he made the capital discovery of the reason why

magnets set

in definite

orientations with respect to

the earth ;

which is, that the earth is itself a great magnet, having one of

its poles in high northern and the other in high southern

latitudes. Thus the property of the compass was seen to be

included in the general principle, that the north-seeking pole of

* " Procul dubio oranes lineae hujusmodi in duo puncta concurrent sicut omnes orbes meridian! in duo concurrunt polos mundi oppositos."
t The form in the Colchester records is Gilberd.
J Gulielmi Gilberti de Magnete, Magneticisque corporibus, et de magno magnete tellure : London, 1600. An English translation by P. F. Mottelay was published
in 1893.

in the Seventeenth Century.

9

every magnet attracts the south-seeking pole of every other magnet, and repels its north-seeking pole.

Descartes attempted* to account for magnetic phenomena

by his theory of vortices.

A vortex of fluid matter was

postulated round each magnet, the matter of the vortex entering by one pole and leaving by the other : this matter was supposed to act on iron and steel by virtue of a special resistance to its

motion afforded by the molecules of those substances.

Crude though the Cartesian system was in this and many

other features, there is no doubt that by presenting definite conceptions of molecular activity, and applying them to so wide a range of phenomena, it stimulated the spirit of inquiry, and prepared the way for the more accurate theories that came after. In its own day it met with great acceptance: the confusion which

had resulted from the destruction of the old order was now, as

it seemed, ended by a reconstruction of knowledge in a system

at once credible and complete. Nor did its influence quickly

wane ;

for

even

at Cambridge

it was studied long after Newton

had published his theory of gravitation ;f and in the middle of the eighteenth century Euler and two of the Bernoullis based

the explanation of magnetism on the hypothesis of vertices.*

Descartes' theory of light rapidly displaced the conceptions
which had held sway in the Middle Ages. The validity

of his explanation of refraction was, however, called in

question by his fellow-countryman Pierre de Ferinat (b. 1601, d. 1665), and a controversy ensued, which was kept up by the Cartesians long after the death of their master. Fermat

* Principia, Part iv, 133 sqq. f Winston has recorded that, having returned to Cambridge after his ordination in 1693, he resumed his studies there, " particularly the Mathematicks,
and the Cartesian Philosophy : which was alone in Vogue with us at that Time. But it was not long before I, with immense Pains, but no Assistance, set myself

with the utmost Zeal to the study of Sir Isaac Newton's M-onderful Discoveries." \Vhiston's Memoirs (1749), i, p. 36. J Their memoirs shared a prize of the French Academy in 1743, and were

printed in 1752 in the Heciieil des pieces qui ontremporte les prix de VAcad., tome v.

Renati

Descartes Epistolae,

Pars

tertia ;

Amstelodami, 1683.

The Fennat

correspondence is comprised in letters xxix to XLVI.

10

The Theory of the Aether

eventually introduced a new fundamental law, from which he

proposed to deduce the paths of rays of light. This was the

celebrated Principle of Least Time, enunciated* in the form,
" Nature always acts by the shortest course." From it the law

of reflexion can readily be derived, since the path described by

light between a point 011 the incident ray and a point on the

reflected ray is the shortest possible consistent with the con-

dition of meeting the reflecting surfaces. t In order to obtain the law of refraction, Fermat assumed that " the resistance of the

media is different," and applied his "method of maxima and minima " to find the path which would be described in the least

time from a point of one medium to a point of the other. In

1661 he arrived at the solution.* "The result of my work," he
writes, " has been the most extraordinary, the most unforeseen,

and the happiest, that ever was ; for, after having performed all

the equations, multiplications, antitheses, and other operations

of my method, and having finally finished the problem, I have

found that my principle gives exactly and precisely the same

proportion for the refractions which Monsieur Descartes has

established." His surprise was all the greater, as he had supposed light to move more slowly in dense than in rare media,

whereas Descartes had (as will be evident from the demonstration

given above) been obliged to make the contrary supposition.

Although Fermat's result was correct, and, indeed, of high

permanent interest, the principles from which it was derived

were metaphysical rather than physical in character, and con-

sequently were of little use for the purpose of framing a

mechanical explanation of light. Descartes' theory therefore

held the field until the publication in 1667 of the Micrographics

* Epist. XLII,

written at Toulouse

in

August,

1657,

to Monsieur de

la

Chambre ;

reprinted in (Euvres de Fermat (ed.

1891), ii, p. 354.

t That reflected light follows the shortest path was no new result, for it had

been affirmed (and attributed to Hero of Alexandria) in the Ke<t>aA.cua rwv OTTTIKUHT

of Heliodorns of Larissa, a work of which several editions were published in the

seventeenth, century.

J Epist.

XLIII,

written at

Toulouse

on Jan.

1,

1662 ;

reprinted

in

(Euvres

de

Fermat, ii, p. 457 ; i, pp. 170, 173. The imprimatur of Viscount Brouncker, P.R.S., is dated Nov. 23, 1664.

in the Seventeenth Centnry.

11

of Eobert Hooke (b. 1635, d. 1703), one of the founders of the Eoyal Society, and at one time its Secretary.
Hooke, who was both an observer and a theorist, made two

experimental discoveries which concern our present subject ; but in both of these, as it appeared, he had been anticipated. The first* was the observation of the iridescent colours which are

seen when light falls on a thin layer of air between two glass plates or lenses, or on a thin film of any transparent substance. These are generally known as the " colours of thin plates," or " Newton's rings " ; they had been previously observed by Boyle.f Hooke's second experimental discovery,^ made after the date of
the Micrographia, was that light in air is not propagated exactly in straight lines, but that there is some illumination within the geometrical shadow of an opaque body. This observation had been published in 1665 in. a posthumous work of Francesco Maria Grimaldi (b. 1618, d. 1663), who had given to the phenomenon the name diffraction.
Hooke's theoretical investigations on light were of great
importance, representing as they do the transition from the

Cartesian system to the fully developed theory of undulations.

He begins by attacking Descartes' proposition, that light is a

tendency to motion rather than an actual motion. " There is,"

he observes, 1 1 " no luminous Body but has the parts of it in

motion

more

or

" less
;

and this motion is

"
exceeding

quick."

Moreover, since some bodies (e.g. the diamond when rubbed or

heated in the dark) shine for a considerable time without being

wasted away, it follows that whatever is in motion is not per-

manently lost to the body, and therefore that the motion must

be of a to-and-fro or vibratory character. The amplitude of the

vibrations must be exceedingly small, since some luminous bodies

(e.g. the diamond again) are very hard, and so cannot yield or

bend to any sensible extent.

* Micrographia, p. 47.

t Boyle's Works (ed. 1772), i, p. 742.

% Hooke's Posthumous Works, p. 186.

Pkysico- Mathesis de lumine, coloribits, et iride.

Bologna,

1665

;

book i,

prop. i.

|| Micrographia, p. 55.

12

The Theory of the Aether

Concluding, then, that the condition associated with the

emission of light by a luminous body is a rapid vibratory motion

of very small amplitude, Hooke next inquires how light travels

through space. " The next thing we are to consider," he says,

"
is

the

way

or

manner

of

the

trajection

of

this motion

through

the interpos'd pellucid body to the eye : And here it will be

easily granted
" First, that it must be a body susceptible and impartible of this motion that will deserve the name of a Transparent ; and next, that the parts of such a body must be homogeneous, or of the same kind.
" Thirdly, that the constitution and motion of the parts must be such that the appulse of the luminous body may be communicated or propagated through it to the greatest imaginable distance in the least imaginable time, though I see no reason to affirm that it must be in an instant.
" Fourthly, that the motion is propagated every way through an Homogeneous medium by direct or straight lines extended every way like Eays from the centre of a Sphere.
" Fifthly, in an Homogeneous medium this motion is propagated every way with equal velocity, whence necessarily every pulse or vibration of the luminous body will generate a Sphere, which will continually increase, and grow bigger, just after the same manner (though indefinitely swifter) as the waves or rings on the surface of the water do swell into bigger and bigger circles about a point of it, where by the sinking of a Stone the motion was begun, whence it necessarily follows, that all the parts of these Spheres undulated through an Homogeneous medium cut the Kays at right angles."
Here we have a fairly definite mechanical conception. It resembles that of Descartes in postulating a medium as the
vehicle of light ; but according to the Cartesian hypothesis the disturbance is a statical pressure in this medium, while in Hooke's theory it is a rapid vibratory motion of small amplitude.
In the above extract Hooke introduces, moreover, the idea of
the wave-swrface, or locus at any instant of a disturbance gene-

in the Seventeenth Century.

13

rated originally at a point, and affirms that it is a sphere, whose, centre is the point in question, and whose radii are

the rays of light issuing from the point.

Hooke's next effort was to produce a mechanical theory of

refraction, to replace that given by Descartes.

"
Because,"

he

says, "all transparent mediums are not Homogeneous to one another, therefore we will next examine how this pulse or motion

will be propagated through differingly transparent mediums.

And here, according to the most acute and excellent Philosopher

Des Cartes, I suppose the sine of the angle of inclination in the

first medium to be to the sine of refraction in the second, as the

density of the first to the density of the second. By density, I mean not the density in respect of gravity (with which the
refractions or transparency of mediums hold no proportion), but
in respect only to the trajeetion of the Kays of light, in which respect they only differ in this, that the one propagates the pulse more easily and weakly, the other more slowly, but
more strongly. But as for the pulses themselves, they will by the refraction acquire another property, which we shall now

endeavour to explicate.

"We ACFD will suppose, therefore, in the first Figure,

to be

a physical Kay, or ABC and DEFto be two mathematical Kaysr
trajected from a very remote point of a luminous body through

14

The Theory of the Aether

an Homogeneous transparent medium LL, and DA, EB, FC, to be

small portions of the orbicular impulses which must therefore

cut the Rays at right angles : these Rays meeting with the plain

NO surface

of a medium that yields an easier transitus to the

propagation of light, and falling obliquely on it, they will in the

MM medium

be refracted towards the perpendicular of the

surface. And because this medium is more easily trajected than

the former by a third, therefore the point of the orbicular
pulse FG will be moved to If four spaces in the same time that

F, the other end of it, is moved to three spaces, therefore the
H whole refracted pulse to shall be oblique to the refracted Rays
GHK and /."

Although this is not in all respects successful, it represents

a decided advance on the treatment of the same problem by Descartes, which rested on a mere analogy. Hooke tries to

determine what happens to the wave-front when it meets

the

interface

between

two

media ;

and

for this end he

intro-

duces the correct principle that the side of the wave-front

which first meets the interface will go forward in the second
medium with the velocity proper to that medium, while the

other side of the wave-front which is still in the first medium

is still moving with the old velocity : so that the wave-front will be deflected in the transition from one medium to the

other.

This deflection of the wave-front was supposed by Hooke to
be the origin of the prismatic colours. He regarded natural or
white light as the simplest type of disturbance, being constituted by a simple and uniform pulse at right angles to the direction of propagation, and inferred that colour is generated by the distortion to which this disturbance is subjected in the process of refraction. "The Ray,"* he says, " is dispersed, split, and
opened by its Refraction at the Superficies of a second medium, and from a line is opened into a diverging Superficies, and
so obliquated, whereby the appearances of Colours are produced."

* Hooke, Posthnmo/is Works, p. 82.

in the Seventeenth Century.

\5

" Colour/'

he

says

in

another

place,*

" is

nothing

but

the

disturbance of light by the communication of the pulse to other transparent mediums, that is by the refraction thereof." His

precise hypothesis regarding the different colours wasf "that Blue is an impression on the Retina of an oblique and confus'd pulse of light, whose weakest part precedes, and whose strongest follows. And, that red is an impression on the Retina of an oblique and confus'd pulse of light, whose strongest part

precedes, and whose weakest follows." Hooke's theory of colour was completely overthrown, within

a few years of its publication, by one of the earliest discoveries of Isaac Xewton (b. 1642, d. 1727). Newton, who was elected
a Fellow of Trinity College, Cambridge, in 1667, had in the beginning of 1666 obtained a triangular prism, " to trytherewith the celebrated Phaenomena of Colours." For this

purpose, " having darkened my chamber, and made a small hole

in my window-shuts, to let in a convenient quantity of the

Sun's light, I placed my Prisme at his entrance, that it might

be thereby refracted to the opposite wall. It was at first a

very pleasing divertisement, to view the vivid and intense

colours produced thereby ; but after a while applying myself to

consider them more circumspectly, I became surprised to see

them in an oblong form, which, according to the received laws

of Refraction, I expected should have been circular" The

length of the coloured spectrum was in fact about five times as

great as its breadth.

This puzzling fact he set himself to study ; and after more

experiments the true explanation was discovered namely,

that ordinary white light is really a mixture of rays of every variety of colour, and that the elongation of the spectrum is

due to the differences in the refractive power of the glass for

these different rays.
" Amidst these thoughts," he tells us,+ " I was forced from

*To the Royal Society, February 15, 1671-2.
t Micrographia, p. 64. J Phil. Trans., Xo. 80, February 19, 1671-2.

16

The Theory of the Aether

"
Cambridge by the intervening Plague ; this was in 1666, and

his memoir on the subject was not presented to the Koyal

Society until five years later. In it he propounds a theory of

colour directly opposed to that of Hooke.

" Colours,"

he

says,

"are not Qualifications of light derived from Refractions, or

Reflections of natural Bodies (as 'tis generally believed), but

Original and connate properties, which in divers Rays are divers.

Some Rays are disposed to exhibit a red colour and no other :

some a yellow and no other, some a green and no other, and so

of the rest. Nor are there only Rays proper and particular to

the more eminent colours, but even to all their intermediate

gradations.
" To the same degree of Refrangibility ever belongs the same colour, and to the same colour ever belongs the same
degree of Refrangibility."
" The species of colour, and degree of Refrangibility proper to any particular sort of Rays, is not mutable by Refraction, nor by Reflection from natural bodies, nor by, any other cause, that
I could yet observe. When any one sort of Rays hath been
well parted from those of other kinds, it hath afterwards
obstinately retained its colour, notwithstanding my utmost
endeavours to change it."
The publication of the new theory gave rise to an acute controversy. As might have been expected, Hooke was foremost among the opponents, and led the attack with some degree of
asperity. When it is remembered that at this time Newton
was at the outset of his career, while Hooke was an older man,
with an established reputation, such harshness appears particularly ungenerous; and it is likely that the unpleasant consequences which followed the announcement of his first
great discovery had much to do with the reluctance which Newton ever afterwards showed to publish his results to the
world.
In the course of the discussion Newton found occasion to
explain more fully the views which he entertained regarding the nature of light. Hooke charged him with holding the

in the Seventeenth Century.

17

doctrine that light is a material substance. Now Newton had, as

a matter of fact, a great dislike of the more imaginative kind of

hypotheses ; he altogether renounced the attempt to construct

the universe from its foundations after the fashion of Descartes,

and aspired to nothing more than a formulation of the laws

which directly govern the actual phenomena. His theory of gravitation, for example, is strictly an expression of the results of observation, and involves no hypothesis as to the cause of the

attraction

which

subsists

between

ponderable

bodies ;

and his

own desire in regard to optics was to present a theory free from

speculation as to the hidden mechanism of light. Accordingly, in reply to Hooke's criticism, he protested* that his views on

colour were in no way bound up with any particular conception

of the ultimate nature of optical processes.
Xewton was, however, unable to carry out his plan of
connecting together the phenomena of light into a coherent and reasoned whole without having recourse to hypotheses. The hypothesis of Hooke, that light consists in vibrations of an aether, he rejected for reasons which at that time were perfectly cogent, and which indeed were not successfully refuted for over a century. One of these was the incompetence of the wavetheory to account for the rectilinear propagation of light, and another was its inability to embrace the facts discovered, as we shall presently see, by Huygens, and first interpreted
correctly by Newton himself of polarization. On the whole, he seems to have favoured a scheme of which the following may be taken as a summaryf :
All space is permeated by an elastic medium or aether, which is capable of propagating vibrations in the same way as the

*Phil. Trans, vii, 1672, p. 5086. t Cf. Newton's memoir in Phil. Trans, vii, 1672 ; his memoir presented to the Royal Society in December, 1675, which is printed in Birch, iii, p. 247; his Opticks, especially Queries 18, 19, 20, 21, 23, 29; the Scholium at the end of the Principia ; and a letter to Boyle, written in February, 1678-9, which is printed in Horsley's Newtoni Opera, p. 385.
In the Principia, Book I., section xiv, the analogy between rays of light and streams of corpuscles is indicated ; but Newton does not commit himself to any theory of light based on this.
C

18

The Theory of the Aether

air propagates the vibrations of sound, but with far greater

velocity.

This aether pervades the pores of all material bodies, and

is

the cause of their

cohesion ;

its density varies from

one

body

to another, being greatest in the free interplanetary spaces. It

is not necessarily a single uniform substance : but just as air

contains aqueous vapour, so the aether may contain various " aethereal spirits," adapted to produce the phenomena of

electricity, magnetism, and gravitation.

The vibrations of the aether cannot, for the reasons already

mentioned, be supposed in themselves to constitute light.

Light

is

therefore

taken

to

be

"
something

of

a

different

kind,

propagated from lucid bodies. They, that will, may suppose

it an aggregate of various peripatetic qualities. Others may

suppose it multitudes of unimaginable small and swift

corpuscles of various sizes, springing from shining bodies

at great distances one after another; but yet without any

sensible interval of time, and continually urged forward by a

principle of motion, which in the beginning accelerates them,

till the resistance of the aethereal medium equals the force of that principle, much after the manner that bodies let fall in

water are accelerated till the resistance of the water equals the

force of gravity. But they, that like not this, may suppose

light any other corporeal emanation, or any impulse or motion of any other medium or aethereal spirit diffused through the

main body of aether, or what else they can imagine proper for

this purpose. To avoid dispute, and make this hypothesis general, let every man here take his fancy ; only whatever

light be, I suppose it consists of rays differing from one another

in contingent circumstances, as bigness, form, or vigour."*

In any case, light and aether are capable of mutual inter-

action; aether is in fact the intermediary between light and
ponderable matter. When a ray of light meets a stratum of

aether denser or rarer than that through which it has lately

been passing, it is, in general, deflected from its rectilinear

* Royal

Society,

Dec. 9,

1675.

in the Seventeenth Century.

19

course ; and differences of density of the aether between one

material medium and another account on these principles for

the reflexion and refraction of light. The condensation or

rarefaction of the aether due to a material body extends to

some little distance from the surface of the body, so that the

inflexion due to it is really continuous, and not abrupt; and

this further explains diffraction, which Newton took to be

"
only

a

new

kind of refraction, caused, perhaps, by the

external aethers beginning to grow rarer a little before it

came at the opake body, than it was in free spaces."

Although the regular vibrations of Newton's aether were not

supposed to constitute light, its irregular turbulence seems to

have represented fairly closely his conception of heat. He

supposed that when light is absorbed by a material body,

vibrations are set up in the aether, and are recognizable as

the heat which is always generated in such cases. The

conduction of heat from hot bodies to contiguous cold ones he

conceived to be effected by vibrations of the aether propagated

between them ;

and he supposed that it

is the violent

agitation

of aethereal motions which excites incandescent substances to

emit light.

Assuming with Newton that light is not actually con-

stituted by the vibrations of an aether, even though such

vibrations may exist in close connexion with it, the most

definite and easily conceived supposition is that rays of light

are streams of corpuscles emitted by luminous bodies. Although

this was not the hypothesis of Descartes himself, it was so

thoroughly akin to his general scheme that the scientific men

of Newton's generation, who were for the most part deeply

imbued with the Cartesian philosophy, instinctively selected

it from the wide choice of hypotheses which Newton had offered

them ;

and

by later

writers

it

was

generally

associated

with

A Newton's name.

curious argument in its favour was drawn

from a phenomenon which had then been known for nearly half

a century : Vincenzo Cascariolo, a shoemaker of Bologna, had

discovered, about 1630, that a substance, which afterwards

C2

20

The Theory of the Aether

received the name of Bologna stone or Bologna phosphorus, has-
the property of shining in the dark after it has been exposed for some time to sunlight ; and the storage of light which seemed to be here involved was more easily explicable on the corpuscular theory than on any other. The evidence in
this quarter, however, pointed the other way when it was
found that phosphorescent substances do not necessarily emit the same kind of light as that which was used to stimulate
them.

In accordance with his earliest discovery, Newton considered colour to be an inherent characteristic of light, and inferred that it must be associated with some definite quality of the corpuscles or aether-vibrations. The corpuscles corresponding
to different colours would, he remarked, like sonorous bodies of

different pitch, excite vibrations of different types in the

aether ;

and

" if

by any

means those

[aether- vibrations]

of

unequal bignesses be separated from one another, the largest

beget a Sensation of a Red colour, the least or shortest of a

deep Violet,

and

the

intermediate

ones,

of

intermediate

colours ;

much after the manner that bodies, according to their several

sizes, shapes, and motions, excite vibrations in the Air of various

bignesses, which, according to those bignesses, make several

Tones in Sound."*

This sentence is the first enunciation of the great principle
that homogeneous light is essentially periodic in its nature, and
that differences of period correspond to differences of colour.
The analogy with Sound is obvious ; and it may be remarked
in passing that Newton's theory of periodic vibrations in an elastic medium, which he developed! in connexion with the
explanation of Sound, would alone entitle him to a place among those who have exercised the greatest influence on the theory of light, even if he had made no direct contribution to the
latter subject.

* Phil. Trans, vii (1672), p. 5088. t Newton's Prmcipia, Book ii., Props, xliii.-l.

in the Seventeenth Century.

21

Newton devoted considerable attention to the colours of

thin, plates, and determined the empirical laws of the

phenomena with great accuracy. In order to explain them, he

supposed

that

"
every ray

of

light,

in

its

passage through any

refracting surface, is put into a certain transient constitution or

state, which, in the progress of the ray, returns at equal

intervals, and disposes the ray, at every return, to be easily

transmitted through the next refracting surface, and, between the returns, to be easily reflected by it."* The interval

between two consecutive dispositions to easy transmission, or

"
length

of

fit,"

he

supposed

to

depend

on

the

colour,

being

greatest for red light and least for violet. If then a ray of

homogeneous light falls on a thin plate, its fortunes as regards transmission and reflexion at the two surfaces will depend on

the relation which the length of fit bears to the thickness of

the plate ; and on this basis he built up a theory of the colours

of thin plates. It is evident that Newton's "length of fit" corresponds in some measure to the quantity which in the undulatory theory is called the wave-length of the light ; but the suppositions of easy transmission and reflexion were soon

found inadequate to explain all Newton's experimental results

.at least without making other and more complicated additional

assumptions.

At the time of the publication of Hooke's Micrographia, and

Newton's theory of colours, it was not known whether light

is propagated instantaneously or not. An attempt to settle

the question experimentally had been made many years

previously by Galileo,f who had stationed two men with

lanterns

at

a

considerable

distance

from

each

other ;

one

of

them was directed to observe when the other uncovered his

light, and exhibit his own the moment he perceived it. But the interval of time required by the light for its journey was too small to be perceived in this way ; and the discovery was

* Optic ks, Book ii., Prop. 12. t Discorri e dimostrazioiti matemaliche, p. 43 of the Elzevir edition of 1638.

22

The Theory of the Aether

ultimately made by an astronomer. It was observed in 1675 by Olof Roemer* (b. 1644, d. 1710) that the eclipses of the first satellites of Jupiter were apparently affected by an unknown disturbing cause ; the time of the occurrence of the phenomenon was retarded when the earth and Jupiter, in the course of their orbital motions, happened to be most remote from each other, and accelerated in the contrary case. Eoemer explained this by supposing that light requires a finite time for its propagation from the satellite to the earth ; and by observations of
eclipses, he calculated the interval required for its passage from the sun to the earth (the light-equation, as it is called) to be 11 minutes,f
Shortly after Roemer's discovery, the wave-theory of light
was greatly improved and extended by Christiaan Huygens (b. 1629, d. 1695). Huygens, who at the time was living in Paris, communicated his results in 1678 to Cassini, Eoemer,
De la Hire, and the other physicists of the French Academy,
and prepared a manuscript of considerable length on the subject. This he proposed to translate into Latin, and to publish in that language together with a treatise on the Optics of Telescopes ; but the work of translation making little progress, after a delay of twelve years, he decided to print the work on wave-theory in its original form. In 1690 it appeared at Leyden,J under the title Traite de la lumiere ou sont expliquees les causes de ce qui luy arrive dans la reflexion et dans la refraction. Et parti-

*Mem. de 1'Acad. x. (1666-1699), p. 575.

t It was soon recognized that Roemer's value was too large ; and the

astronomers of the succeeding half-century reduced it to 7 minutes. Delambre,

by an investigation whose details appear to have been completely destroyed,

published in 1817 the value

493 -2 s from a ,

discussion

of

eclipses

of

Jupiter's

satellites during the previous 150 years. Glasenapp, in an inaugural dissertation

published in 1875, discussed the eclipses of the first satellite between 1848 and

1870, and derived,

by

different

assumptions,

values

between

496 s

and

501 s ,

the

most probable value being 500-88. Sampson, in 1909, derived 498'64S from his
own readings of the Harvard Observations, and 498'79 S from the Harvard readings,

with probable errors of about + 0'02". The inequalities of Jupiter's surface give

rise to some difficulty in exact determinations.

% Huygens had by this time returned to Holland.

in the Seventeenth Century.

23

culierement dans Vetrange refraction du cristal d'Islande. Par
C.ff.D.Z*

The truth of Hooke's hypothesis, that light is essentially a

form of motion, seemed to Huygens to be proved ]}y the effects

observed with burning-glasses ; for in the combustion induced at

the focus

of

the glass, the

molecules of bodies

are

dissociated ;

which, as he remarked, must be taken as a certain sign of motion,

if, in conformity to the Cartesian philosophy, we seek the cause

of all natural phenomena in purely mechanical actions.

The question then arises as to whether the motion is that

of a medium, as is supposed in Hooke's theory, or whether it

may be compared rather to that of a flight of arrows, as in the

corpuscular theory. Huygens decided that the former alter-

native is the only tenable one, since beams of light proceeding

in directions inclined to each other do not interfere with each

other in any way. Moreover, it had previously been shown by Torricelli that
light is transmitted as readily through a vacuum as through air ; and from this Huygens inferred that the medium or aether in which the propagation takes place must penetrate all matter, and be present even in all so-called vacua.
The process of wave-propagation he discussed by aid of a principle which was nowf introduced for the first time, and has
since been generally known by his name. It may be stated
thus : Consider a wave-front,* or locus of disturbance, as it
exists at a definite instant t : then each surface-element of the

wave-front may be regarded as the source of a secondary wave, which in a homogeneous isotropic medium will be propagated
outwards from the surface-element in the form of a sphere whose radius at any subsequent instant t is proportional to (t-t ) ; and the wave-front which represents the whole distur-

*
i.e.

Cbristiaan Huygens de Zuylichem.

The custom of indicating names by

initials was not unusual in that age.

t Traite de la lum., p. 17.
I It maybe remarked that Huygens' " waves " are really what modern writers, following Hooke, call " pulses "; Huygens never considered true wave-trains

having the property of periodicity.

24

The Theory of the Aether

bance at the instant t is simply the envelope of the secondary waves which arise from the various surface elements of the

original wave-front.* The introduction of this principle enabled Huygens to succeed where Hooke and other contemporary wave-theoristsf had failed, in achieving the explanation of refraction and reflexion. His method was to combine his own

principle with Hooke's device of following separately the fortunes

of the right-hand and left-hand sides of a wave-front when it

reaches the interface between two media. The actual explana-

tion for the case of reflexion is as follows :

AB Let

represent the interface at which reflexion takes

AHC GMB place,

the incident wave-front at an instant ,

the

position which the wave-front would occupy at a later instant t

if the propagation were not interrupted by reflexion. Then by

"G

A Huygens' principle the secondary wave from is at the instant

ENS AG H t a sphere

of radius equal to

: the disturbance from t

after meeting the interface at K, will generate a secondary

TV wave

oi radius equal to KM, and similarly the secondary

wave corresponding to any other element of the original wave-

* The justification for this was given long afterwards by Fresnel, Annales de
chimie, xxi.
t e.g. Ignace Gaston Pardies and Pierre Ango, the latter of whom published
a work on Optics at Paris'in 1682.

in the Seventeenth Century.

25

front can be found. It is obvious that the envelope of these

secondary waves, which constitutes the final wave-front, will be

AB a plane BN, which will be inclined to

at the same angle as

AC. This gives the law of reflexion.

The law of refraction is established by similar reasoning,

on the supposition that the velocity of light depends on the

medium in which it is propagated. Since a ray which passes

from air to glass is bent inwards towards the normal, it may be

inferred that light travels more slowly in glass than in air.

Huygens offered a physical explanation of the variation in

velocity of light from one medium to another, by supposing

that transparent bodies consist of hard particles which interact

with the aethereal matter, modifying its elasticity. The

opacity of metals he explained by an extension of the same

idea, supposing that some of the particles of metals are hard

(these account for reflexion) and the rest soft : the latter destroy

the luminous motion by damping it.

The second half of the Theorie de la lumiere is concerned with

a phenomenon which had been discovered a few years pre-

viously by a Danish philosopher, Erasmus Bartholin (b. 1625,

A d. 1698).

sailor had brought from Iceland to Copenhagen a

number of beautiful crystals which he had collected in the Bay

of Eoerford. Bartholin, into whose hands they passed, noticed*

that any small object viewed through one of these crystals

appeared double, and found the immediate cause of this in the

fact that a ray of light entering the crystal gave rise in general
to two refracted rays. One of these rays was subject to the ordinary law of refraction, while the other, which was called the extraordinary ray, obeyed a different law, which Bartholin
did not succeed in determining.
The matter had arrived at this stage when it was taken up by Huygens. Since in his conception each ray of light corresponds to the propagation of a wave-front, the two rays in Iceland spar must correspond to two different wave-fronts propagated

* Ejcperimenta cristatti Islandici disdiaclastici : 1669.

26

The Theory of the Aether

simultaneously. In this idea he found no difficulty ; as he says : " It is certain that a space occupied by more than one kind of
matter may permit the propagation of several kinds of waves,
different in velocity; for this actually happens in air mixed with aethereal matter, where sound-waves and light-waves are

propagated together."

Accordingly he supposed that a light-disturbance generated

at any spot within a crystal of Iceland spar spreads out in the

form of a wave-surface, composed of a sphere and a spheroid

having the origin of disturbance as centre. The spherical wave-

front corresponds to the ordinary ray, and the spheroid to the

extraordinary ray ; and the direction in which the extraordinary

ray is refracted may be determined by a geometrical construc-

tion, in which the spheroid takes the place which in the

ordinary construction is taken by the sphere.

Thus, let the plane of the figure be at right angles to the

intersection of the wave-front with the surface of the crystal ;

AB let

represent

the

trace

of

the

incident

wave-front ;

and

B suppose that in unit time the disturbance from reaches the

interface at T. In this unit-interval of time the disturbance

A from will have spread out within the crystal into a sphere

and spheroid : so the wave-front corresponding to the

ordinary ray will be the tangent-plane to the sphere through the line whose trace is T, while the wave-front corresponding to the extraordinary ray will be the tangent-plane to the
N spheroid through the same line. The points of contact

in the Seventeenth Century.

27

M AN M and

will determine the directions

A and

of the two-

refracted rays* within the crystal.
Huygens did not in the Thtoi-ie de la lumiere attempt a detailed physical explanation of the spheroidal wave, but communicated one later in a letter to Papin,f written in December, 1690. " As to the kinds of matter contained in Iceland crystal," he says, " I suppose one composed of small spheroids, and another which occupies the interspaces around these spheroids, and which serves to bind them together. Besides these, there is the matter of aether permeating all the crystal, both between and within the parcels of the two kinds of matter just mentioned ; for I suppose both the little spheroids, and the matter which occupies the intervals around them, to be composed of small fixed particles, amongst which are diffused in perpetual motion the still finer particles of the aether. There is now no reason why the ordinary ray in the crystal should not be due to waves propagated in this aethereal matter. To account for the extraordinary refraction, I conceive another kind of waves, which have for
vehicle both the aethereal matter and the two other kinds of

matter constituting the crystal. Of these latter, I suppose that the matter of the small spheroids transmits the waves a little more quickly than the aethereal matter, while that around the spheroids transmits these waves a little more slowly than the same aethereal matter. . . . These same waves, when they travel in the direction of the breadth of the spheroids, meet with more of the matter of the spheroids, or at least pass with less obstruction, and so are propagated a little more quickly in this sense than in the other ; thus the light-disturbance is propagated as a spheroidal sheet."
Huygens made another discoveryj of capital importance when

* The word ray in the wave-theory is always applied to the line which goes from the centre of a wave (i.e. the origin of the disturbnnce) to a point on its surface, whatever may be the inclination of this line to the surface-element on which it abuts; for this line has the optical properties of the "rays" of the
emission theory.
t Huygens' (Envres, ed. 1905, x., p. 177.
+ T/ieorie de la lumiere, p. 89.

28 Theory of the Aether in the Seventeenth Century.

experimenting with the Iceland crystal. He observed that the

two rays which are obtained by the double refraction of a single

ray afterwards behave in a way different from ordinary light

which has not experienced double

refraction ;

and in particular,

if one of these rays is incident on a second crystal of Iceland

spar, it gives rise in some circumstances to two, and in others

to only one, refracted ray. The behaviour of the ray at this

second refraction can be altered by simply rotating the second

crystal

about

the

direction

of

the

ray as

axis ;

the ray under-

going the ordinary or extraordinary refraction according as the

principal section of the crystal is in a certain direction or in the

direction at right angles to this.

The first stage in the explanation of Huygens' observation was reached by Newton, who in 1717 showed* that a ray

obtained by double refraction differs from a ray of ordinary

light in the same way that a long rod whose cross-section is a

rectangle differs from a long rod whose cross-section is a circle :

in other words, the properties of a ray of ordinary light are the

same with respect to all directions at right angles to its direction

of propagation, whereas a ray obtained by double refraction

must be supposed to have sides, or properties related to special directions at right angles to its own direction. The refraction

of such a ray at the surface of a crystal depends on the relation

of its sides to the principal plane of the crystal.

That a ray of light should possess such properties seemed to Newtonf an insuperable objection to the hypothesis which
regarded waves of light as analogous to waves of sound. On

this point he was in the right : his objections are perfectly

valid against the wave-theory as it was understood by his

contemporariesJ, although not against the theory which was put

forward a century later by Young and Fresnel.

* The second edition of Newton's Opticks, Query 26.

t Opticks, Query 28.

J In which the oscillations are performed in the direction in which the wave

advances.

In which the oscillations are performed in a direction at right angles to that in which the wave advances.

29 )

CHAPTEE II.

ELECTRIC AND MAGNETIC SCIENCE PRIOR TO THE INTRODUCTION OF THE POTENTIALS.

THE magnetic discoveries of Peregrinus and Gilbert, and the vortex-hypothesis by which Descartes had attempted to explain them,* had raised magnetism to the rank of a separate science by the middle of the seventeenth century. The kindred science of electricity was at that time in a less developed state ; but it
had been considerably advanced by Gilbert, whose researches in this direction will now be noticed.

For two thousand years the attractive power of amber had

been regarded as a virtue peculiar to that substance, or possessed

by at most one or two others. Gilbert provedf this view to be

mistaken, showing that the same effects are induced by friction

in

quite

a

large

class

of

bodies ;

among

which

he

mentioned

glass, sulphur, sealing-wax, and various precious stones.
A force which was manifested by so many different kinds of

matter seemed to need a name of its own; and accordingly

Gilbert gave to it the name electric, which it has ever since

retained.

Between the magnetic and electric forces Gilbert remarked
many distinctions. The lodestone requires no stimulus of friction
such as is needed to stir glass and sulphur into activity. The lodestone attracts only magnetizable substances, whereas electrified bodies attract everything. The magnetic attraction between two bodies is not affected by interposing a sheet of paper, or a linen cloth, or by immersing the bodies in water j whereas the electric attraction is readily destroyed by screens. Lastly, the magnetic force tends to arrange bodies in definite

*Cf. pp. 7-9.

t De Magnete, lib. ii., cap. 2.

30

Electric and Magnetic Science

orientations ;

while

the

electric

force

merely

tends

to

heap

them

together in shapeless clusters. These facts appeared to Gilbert to indicate that electric
phenomena are due to something of a material nature, which under the influence of friction is liberated from the glass or amber in which under ordinary circumstances it is imprisoned. In support of this view he adduced evidence from other quarters. Being a physician, he was well acquainted with the doctrine that the human body contains various humours or kinds of moisture phlegm, blood, choler, and melancholy, which, as they predominated, were supposed to determine the temper of mind; and when he observed that electrifiable bodies were

almost all hard and transparent, and therefore (according to the ideas of that time) formed by the consolidation of watery liquids,
he concluded that the common menstruum of these liquids must
be a particular kind of humour, to the possession of which the electrical properties of bodies were to be referred. Friction
might be supposed to warm or otherwise excite or liberate the
humour, which would then issue from the body as an effluvium and form an atmosphere around it. The effluvium must, he

remarked, be very attenuated, for its emission cannot be detected

by the senses. The existence of an atmosphere of effluvia round every
electrified body might indeed have been inferred, according to Gilbert's ideas, from the single fact of electric attraction. For he believed that matter cannot act where it is not and hence
;
if a body acts on all surrounding objects without appearing to touch them, something must have proceeded out of it unseen.
The whole phenomenon appeared to him to be analogous to the attraction which is exercised by the earth on falling bodies. For in the latter case he conceived of the atmospheric air as the effluvium by which the earth draws all things downwards to

itself.

Gilbert's theory of electrical emanations commended itself
generally to such of the natural philosophers of the seventeenth
century as were interested in the subject ; among whom were

prior to the Introduction of the Potentials.

31

numbered Niccolo Cabeo (b. 1585, d. 1650), an Italian Jesuit

who was. perhaps the first to observe that electrified bodies repel

as

well

as

attract ;

the

English

royalist

exile, Sir Kenelm

Digby (b. 1603, d. 1665); and the celebrated Robert Boyle

(b. 1627, d. 1691). There were, however, some differences of

opinion as to the manner in which the effluvia acted on the small

bodies and set them in motion towards the excited electric;

Gilbert himself had supposed the emanations to have an inherent tendency to reunion with the parent body ; Digby likened their return to the condensation of a vapour by cooling ; and other writers pictured the effluvia as forming vortices round the attracted bodies in the Cartesian fashion.

There is a well-known allusion to Gilbert's hypothesis in

Newton's Opticks.*
" Let him also tell me, how an electrick body can by friction emit an exhalation so rare and subtle,t and yet so potent, as by
its emission to cause no sensible diminution of the weight of the electrick body, and to be expanded through a sphere, whose diameter is above two feet, and yet to be able to agitate and carry up leaf copper, or leaf gold, at a distance of above a foot from the electrick body ? "
It is, perhaps, somewhat surprising that the Newtonian doctrine of gravitation should not have proved a severe blow to
the emanation theory of electricity ; but Gilbert's doctrine was now so firmly established as to be unshaken by the overthrow of the analogy by which it had been originally justified. It was, however, modified in one particular about the beginning of the eighteenth century. In order to account for the fact that

electrics are not perceptibly wasted away by excitement, the

earlier writers had supposed all the emanations to return

ultimately to the body which

had

emitted

them but ;

the

corpuscular theory of light accustomed philosophers to the

idea of

emissions so subtle as

to cause

no perceptible

loss ;

and

* Query 22. t " Subtlety," says Johnson, " which in its original import means exility of particles, is taken in its metaphorical meaning for nicety of distinction."

32

Electric and Magnetic Science

after the time of Newton the doctrine of the return of the-

electric effluvia gradually lost credit.

Newton died in 1727. Of the expositions of his philosophy

which were published in his lifetime by his followers, one at

least deserves to be noticed for the sake of the insight which

it affords into the state of opinion regarding light, heat, and

electricity in the first half of the eighteenth century. This was

the Physices elementa matlwmatica experimentis confirmata of

Wilhelm Jacob s'Gravesande (b. 1688, d. 1742), published at

Leyden in 1720. The Latin edition was afterwards reprinted

several times, and was, moreover, translated into French and

English : it seems to have exercised a considerable and, on the

whole, well-deserved influence on contemporary thought.

s'Gravesande supposed light to consist in the projection of

corpuscles from luminous bodies to [the eye ; the motion being very swift, as is shown by astronomical observations. Since

many bodies, e.g. the metals, become luminous when they- -are

heated, he inferred that every substance possesses a natural

store of corpuscles, which are expelled when it is heated to

incandescence ;

conversely,

corpuscles

may become

united

to

a

material body ; as happens, for instance, when the body is exposed

to the rays of a fire. Moreover, since the heat thus acquired is

readily conducted throughout the substance of the body, he

concluded that corpuscles can penetrate all substances, however

hard and dense they be.

Let us here recall the ideas then current regarding the
nature of material bodies. From the time of Boyle (1626-1691)

it had been recognized generally that substances perceptible to

the

senses

may

be

either elements

or

compounds

or

mixtures ;

the compounds being chemical individuals, distinct from mere

mixtures of elements. But the substances at that time accepted

as elements were very different from those which are now known

by the name. Air and the calces* of the metals figured in the list, while almost all the chemical elements now recognized were

prior to the Introduction oj the Potentials.

33

omitted from it ; some of them, such as oxygen and hydrogen, because they were as yet undiscovered, and others, such as the metals, because they were believed to be compounds.
Among the chemical elements, it became customary after
the time of Newton to include light-corpuscles.* That some-
thing which is confessedly imponderable should ever have been
admitted into this class may at first sight seem surprising. But
it must be remembered that questions of ponderability counted for very little with the philosophers of the period. Threequarters of the eighteenth century had passed before Lavoisier enunciated the fundamental doctrine that the total weight of the substances concerned in a chemical reaction is the same

after the reaction as before it. As soon as this principle came to be universally applied, light parted company from the true elements in the scheme of chemistry.
We must now consider the views which were held at this

time regarding the nature of heat. These are of interest for our present purpose, on account of the analogies which were set up between heat and electricity.
The various conceptions which have been entertained concerning heat fall into one or other of two classes, according as heat is represented as a mere condition producible in bodies, or as a distinct species of matter. The former view, which is that universally held at the present day, was advocated by the great philosophers of the seventeenth century. Bacon maintained it in
the Novum Organum : " Calor," he wrote, " est niotus expansivus,

cohibitus, et nitens per partes minores."f Boyle+ affirmed that the " Nature of Heat " consists in " a various, vehement, and
intestine commotion of the Parts among themselves." Hooke declared that " Heat is a property of a body arising from the motion or agitation of its parts." And Newton|| asked : " Do not

* Newton himself (Oplicks, p. 349) suspected that light-corpuscles and ponderable matter might be transmuted into each other : much later, Boscovich
(Theoria, pp. 215, 217) regarded the matter of light as a principle or element in the constitution of natural bodies.

t Nov. Org., Lib. n., Aphor. xx.

J Mechanical Production of Heat and Cold.

Micrographia, p. 37.

|| Opticks.
D

34

Electric and Magnetic Science

all fixed Bodies, when heated beyond a certain Degree, emit

light

and shine and ;

is

not

this

Emission

performed by the

"

vibrating Motion of their Parts ? and, moreover, suggested the

converse of this, namely, that when light is absorbed by a

material body, vibrations are set up which are perceived by the

senses as heat.

The doctrine that heat is a material substance was main-

tained in Newton's lifetime by a certain school of chemists. The most conspicuous member of the school was Wilhelm Homberg (b. 1652, d. 1715) of Paris, who* identified heat and light with the sulphureous principle, which he supposed to be one of the primary ingredients of all bodies, and to be present even in the interplanetary spaces. Between this view and that of Newton it might at first seem as if nothing but sharp opposition was to be expected,j- But a few years later the professed exponents of the Principia and the Opticks began to develop their system under

the evident influence of Homberg's writings. This evolution

may easily be traced in s'Gravesande, whose starting-point is

the admittedly Newtonian idea that heat bears to light a

relation similar to that which a state of turmoil bears to regular

rectilinear

motion ;

whence, conceiving

light

as a projection

of

corpuscles, he infers that in a hot body the material particles

and the light-corpusclesj are in a state of agitation, which

becomes more violent as the body is more intensely heated.

s'Gravesande thus holds a position between the two opposite
camps. On the one hand he interprets heat as a mode of

motion ;

but on the other

he

associates

it with the presence

of

a particular kind of matter, which he further identifies with the

matter of light. After this the materialistic hypothesis made

* Mem. del'Acad., 1705, p. 88.
t Though it reminds us of a curious conjecture ofNewtoa'i: "Is not the strength and vigour of the action between light and sulphureous bodies one reason M-liy sulphureous bodies take fire more readily and burn more vehemently than other bodies do? "
J I have thought it best to translate s'Gravesande's ignis by " light-corpuscles." This is, I think, fully justified by such of his statements as Quando ignis per lineas rectas oculos nostros intrat, ex motu gttein fibris in fundo oculi cont/tninicai ideam luminis excitat.

prior to the Introduction of the Potentials.

35

rapid progress. It was frankly advocated by another member of the Dutch school, Hermann Boerhaave* (6. 1668, d. 1738),

Professor in the University of Leyden, whose treatise on

chemistry was translated into English in 1727.

Somewhat later it was found that the heating effects of the

rays from incandescent bodies may be separated from their

luminous effects by passing the rays through a plate of glass,

which transmits the light, but absorbs the heat. After this

discovery it was no longer possible to identify the matter of heat

with the corpuscles of light ; and the former was consequently

accepted as a distinct element, under the name of caloric.^ In

the latter part of the eighteenth and early part of the nineteenth

centuries} caloric was generally conceived as occupying the

interstices between the particles of ponderable matter an idea

which fitted in well with the observation that bodies commonly

expand when they are absorbing heat, but which was less com-

petent to explain the fact that water expands when freezing.

The latter difficulty was overcome by supposing the union

between a body and the caloric absorbed in the process of

melting to be of a chemical nature; so that the consequent

changes in volume would be beyond the possibility of prediction.

As we have already remarked, the imponderability of heat

did not appear to the philosophers of the eighteenth century to

be a sufficient reason for excluding it from the list of chemical

elements ;

and

in

any case

there

was

considerable

doubt

as to

whether caloric was ponderable or not. Some experimenters

believed that bodies were heavier when cold than when hot;

others that they were heavier when hot than when cold. The

century was far advanced before Lavoisier and Eumford finally

* Boerhaave followed Homberg in supposing the matter of heat to be present ia
all so-called vacuous spaces. t Scheele in 1777 supposed caloric to be a compound of oxygen and phlogiston,
and light to be oxygen combined with a greater proportion of phlogiston. J In suite of the experiments of Benjamin Thompson, Count Eumford (b. 1753,
.d. 1814), in the closing years of the eighteenth century. These should have
-sufficed to re-establish the older conception of heat.
This had been known since the time of Boyle.
D2

36

Electric and Magnetic Science

proved that the temperature of a body is without sensible influence on its weight.
Perhaps nothing in the history of natural philosophy is more amazing than the vicissitudes of the theory of heat. The true hypothesis, after having met with general acceptance throughout a century, and having been approved by a succession of illustrious men, was deliberately abandoned by their successors

in favour of a conception utterly false, and, in some of its developments, grotesque and absurd.
We must now return to s'Gravesande's book. The pheno-
mena of combustion he explained by assuming that when a body
is sufficiently heated the light-corpuscles interact with the material particles, some constituents being in consequence sepa-
rated and carried away with the corpuscles as flame and smoke.

This view harmonizes with the theory of calcination which had been developed by Becher and his pupil Stahl at the end of theseventeenth century, according to which the metals were supposed to be composed of their calces and an element phlogiston. The process of combustion, by which a metal is changed into itscalx, was interpreted as a decomposition, in which the phlogiston separated from the metal and escaped into the atmosphere ; while the conversion of the calx into the metal was regarded as a union with phlogiston.*

s'Gravesande attributed electric effects to vibrations induced

in effluvia, which he supposed to be permanently attached to

such bodies as amber.

" Glass,"

he asserted,

" contains

in it,

and

has about its surface, a certain atmosphere, which is excited by Friction and put into a vibratory motion ; for it attracts and

* The correct idea of combustion had been advanced by Hooke. "The disso-

lution of inflammable bodies,"

he

asserts in

the Micrographia,

" is performed

by a

substance inherent in and mixed with the air, that is like, if not the very same

with, that which is fixed in saltpetre." But this statement met with little favour

at the time, and the doctrine of the compound nature of metals survived in full

vigour until the discovery of oxygen by Priestley and Scheele in 1771-5. In 1775

Lavoisier reaffirmed Hooke's principle that a metallic calx is not the metal minus

phlogiston, but the metal plus oxygen; and this idea, which carried with it the

recognition of the elementary nature of metals, was generally accepted by the end'

of the eighteenth century.

prior to the Introduction of the Potentials.

37

repels light Bodies. The smallest parts of the glass are agitated by the Attrition, and by reason of their elasticity, their motion is

vibratory, which is communicated to the Atmosphere abovementioned : and therefore that Atmosphere exerts its action the

further, the greater agitation the Parts of the Glass receive when

a greater attrition is given to the glass."
The English translator of s'Gravesande's work was himself

destined to play a considerable part in the history of electrical

science. Jean Theophile Desaguliers (b. 1683, d. 1744) was an Englishman only by adoption. His father had been a Huguenot pastor, who, escaping from France after the revocation of the Edict of Nantes, brought away the boy from La Kochelle, concealed, it is said, in a tub. The young Desaguliers was afterwards ordained,
and became chaplain to that Duke of Chandos who was so

ungratefully ridiculed by Pope. In this situation he formed

friendships with some of the natural philosophers of the capital, and amongst others with Stephen Gray, an experimenter of
whom little is known* beyond the fact that he was a pensioner

of the Charterhouse.

In 1729 Gray communicated, as he says,f " to Dr. Desaguliers

and some other Gentlemen " a discovery he had lately made,

"
showing

that

the

Electrick Vertue

of

a Glass

Tube

may be

conveyed to any other Bodies so as to give them the same

Property of attracting and repelling light Bodies as the Tube does, when excited by rubbing : and that this attractive Vertue

might be carried to Bodies that were many Feet distant from

the Tube."

This was a result of the greatest importance, for previous workers had known of no other way of producing the attractive emanations than by rubbing the body concerned.* It was found

* Those M*ho are interested in the literary history of the eighteenth century will

recall the controversy as to whether the verses on the death of Stephen Gray were

written hy Anna "Williams, whose name they bore, or by her patron Johnson.

| Phil. Trans, xxxvii (1731), pp. 18, 227, 285, 397.

j Otto von Guericke (b. 1602, d. 1686) bad, as a matter of fact, observed the

conduction

of

electricity along a

linen thread ;

but this experiment does not seem

to have been followed up. Cf. Experimenta novamagdeburgica, 1672.

38

Electric and Magonetic Science

that only a limited class of substances, among which the metals were conspicuous, had the capacity of acting as channels for the

transport of the electric power ; to these Desaguliers, who. continued the experiments after Gray's death in 1736, gavfc^ the name non-electrics or conductors.

After Gray's discovery it was no longer possible to believe that the electric effluvia are inseparably connected with the bodies from which they are evoked by rubbing ; and it became necessary to admit that these emanations have an independent existence, and can be transferred from one body to another.
Accordingly we find them recognized, under the name of the
electric fluidft as one of the substances of which the world is constituted. The imponderability of this fluid did not, for the

reasons already mentioned, prevent its admission by the side of light and caloric into the list of chemical elements.
The question was actively debated as to whether the electric fluid was an element sui generis, or, as some suspected, was another manifestation of that principle whose operation is seen in the phenomena of heat. Those who held the latter view urged that the electric fluid and heat can both be induced by friction, can both induce combustion, and can both be transferred

from one body to another by mere contact ; and, moreover, that

the best conductors of heat are also in general the best con-
ductors of electricity. On the other hand it was contended that

the electrification of a body does not cause any appreciable rise

in its temperature; and an experiment of Stephen Gray's

brought to light a yet more striking difference. Gray,J in 1729,.

made two oaken cubes, one solid and the other hollow, and

showed that when electrified in the same way they produced

exactly similar

effects ;

whence

he

concluded

that

it

was only

the surfaces which had taken part in the phenomena. Thus

while heat is disseminated throughout the substance of a body,

the electric fluid resides at or near its surface. In the middle of

* Phil. Trans, xli. (1739), pp. 186, 193, 200, 209: Dissertation concerning

Electricity, 1742.

t The Cartesians defined a fluid to be a body whose minute parts are in a

continual agitation.

J Phil. Trans, xxxvii., p. 35.

prior to the Introduction of the Potentials.

39

the eighteenth century it was generally compared to an enveloping atmosphere. " The electricity which a non-electric of great length (for example, a hempen string 800 or 900 feet long) receives, runs from one end to the other in a sphere of electrical
Effluvia" says Desaguliers in 1740 ^and a report of the French
Academy in 1733 says :f " Around an electrified body there is
formed a vortex of exceedingly fine matter in a state of agitation,, which urges towards the body such light substances as lie within its sphere of activity. The existence of this vortex is
more than a mere conjecture ; for when an electrified body i&
brought close to the face it causes a sensation like that of
encountering a cobweb. "J The report from which this is quoted was prepared in
connexion with the discoveries of Charles-Francois du Fay
(b. 1698, d. 1739), superintendent of gardens to the King of
France. Du Fay accounted for the behaviour of gold leaf when
brought near to an electrified glass tube by supposing that at first the vortex of the tube envelopes the gold-leaf, and so attracts it towards the tube. But when contact occurs, the gold-leaf
acquires the electric virtue, and so becomes surrounded by a vortex of its own. The two vortices, striving to extend in contrary senses, repel each other, and the vortex of the tube, being the stronger, drives away that of the gold-leaf. " It is then certain/' says du Fay,H " that bodies which have become electric by contact are repelled by those which have rendered them electric ; but are they repelled likewise by other electrified
bodies of all kinds ? And do electrified bodies differ from each An other in no respect save their intensity of electrification ? examination of this matter has led me to a discovery which I
should never have foreseen, and of which I believe no one
hitherto has had the least idea."

* Phil. Trans, xli., p. 636.

t Hist, de 1'Acad., 1733, p. 6.

t This observation had been made first by Hawksbee at the beginning of the

century.
Mem. de 1'Acad. des Sciences, 1733, pp. 23, 73, 233, 457 ;
503; 1737, p. 86 ; Phil. Trans, xxxviii. (1734), p. 258.
|| Mem. de 1'Acad., 1733, p. 464.

1734, pp. 341,

40

Electric and Magnetic Science

He found, in fact, that when gold-leaf which had been

electrified by contact with excited glass was brought near to an

excited piece of copal,* an attraction was manifested between

them.

" I had expected," he writes,

"
quite

the opposite effect,

my since, according to

reasoning, the copal and gold-leaf, which

were both electrified, should have repelled each other."

Proceeding with his experiments he found that the gold-leaf,

when electrified and repelled by glass, was attracted by all

electrified resinous substances, and that when repelled by the

We latter it was attracted by the glass. "

see, then," he continues,

" that there are two electricities of a totally different nature

namely, that of transparent solids, such as glass, crystal, &c.,

and that of bituminous or resinous bodies, such as amber, copal,

sealing-wax, &c. Each of them repels bodies which have

contracted an electricity of the same nature as its own, and
We attracts those whose electricity is of the contrary nature.

see even that bodies which are not themselves electrics can

acquire either of these electricities, and that then their effects are similar to those of the bodies which have communicated it

to them."

To the two kinds of electricity whose existence was thus

demonstrated, du Fay gave the names vitreous and resinous, by

which they have ever since been known.

An interest in electrical experiments seems to have spread

XV from du Fay to other members of the Court circle of Louis

;

and from 1745 onwards the Memoirs of the Academy contain a

series of papers on the subject by the Abbe Jean-Antoine Nollet

{&. 1700, d. 1770), afterwards preceptor in natural philosophy

to the Koyal Family. Nollet attributed electric phenomena to

the movement in opposite directions of two currents of a fluid,

"
very

subtle

and

inflammable,"

which

he

supposed

to

be present

in all bodies under all circumstances.f When an electric is

excited by friction, part of this fluid escapes from its pores,

forming an effluent stream; and this loss is repaired by an

A * hard transparent resin, used in the preparation of varnish.
t Cf. Nollet' s lieeherchet, 1749, p. 245.

prior to the Introduction of the Potentials.

41

dtfiucnt stream of the same fluid entering the body from outside. Light bodies in the vicinity, being caught in one or other of these streams, are attracted or repelled from the excited electric.
Nollet's theory was in great vogue for some time ; but six or seven years after its first publication, its author came across a work purporting to be a French translation of a book printed originally in England, describing experiments said to have been made at Philadelphia, in America, by one Benjamin Franklin.
"He could not at first believe," as Franklin tells us in his
AutobiograpJvy, " that such a work came from America, and said it must have been fabricated by his enemies at Paris to decry his system. Afterwards, having been assured that there really existed such a person as Franklin at Philadelphia, which he had doubted, he wrote and published a volume of letters, chiefly addressed to me, defending his theory, and denying the verity
of my experiments, and of the positions deduced from them."
We must now trace the events which led up to the discovery
which so perturbed Nollet. In 1745 Pieter van Musschenbroek (6. 1692, d. 1761),
Professor at Leyden, attempted to find a method of preserving electric charges from the decay which was observed when the charged bodies were surrounded by air. With this purpose he tried the effect of surrounding a charged mass of water by an envelope of some non-conductor, e.g., glass. In one of his experiments, a phial of water was suspended from a gunbarrel by a wire let down a few inches into the water through the cork; and the gun-barrel, suspended on silk lines, was applied so near an excited glass globe that some metallic fringes inserted into the gun-barrel touched the globe in motion. Under these circumstances a friend named Cimaeus, who

happened to grasp the phial with one hand, and touch the gun-

barrel with the

other, received a

violent shock ;

and

it

became

evident that a method of accumulating or intensifying the

electric power had been discovered.*

* The discovery was made independently in the same year by Ewald Georg von Kleist, Dean of Kumrain.

42

Electric and Magno etic Science

Shortly after the discovery of the Leyden phial, as it was

named by Nollet, had become known in England, a London

apothecary named William Watson (6. 1715, d. 1787)* noticed

that when the experiment is performed in this fashion the

observer feels the shock " in no other parts of his body but his

arms

and

" breast

;

whence he

inferred that in the act of

discharge there is a transference of something which takes the

shortest or best- conducting path between the gun-barrel and

the phial. This idea of transference seemed to him to bear

some similarity to Nollet's doctrine of afflux and efflux; and

there can indeed be little doubt that the Abbe's hypothesis,

though totally false in itself, furnished some of the ideas from

which Watson, with the guidance of experiment, constructed

a correct theory. In a memoiirt)read to the Eoyal Society

in October, 1746, he propounded the doctrine that electrical

actions are due to the presence

of

an

"
electrical aether/'

which

in the charging or discharging of a Leyden jar is transferred, but

is not created or destroyed. The excitation of an electric,

according to this view, consists not in the evoking of anything

from within the electric itself without compensation, but in the

accumulation of a surplus of electrical aether by the electric at

the expense of some other body, whose stock is accordingly

depleted. All bodies were supposed to possess a certain natural

store, which could be drawn upon for this purpose. " I have shewn," wrote Watson, " that electricity is the

effect of a very subtil and elastic fluid, occupying all bodies in

contact with the terraqueous globe ; and that every-where, in

its natural state, it is of the same degree of density ; and that

glass

and

other

bodies, which

we

denominate

electrics

per

se y.

have the power, by certain known operations, of taking this fluid

from one body, and conveying it to another, in a quantity

sufficient to be obvious to all our senses; and that, under

* Watson afterwards rose to eminence in the medical profession, and was
knighted.
t Phil. Trans, xliv., p. 718. It may here he noted that it was Watson who improved the phial by coating it nearly to the top, both inside and outside, with
tinfoil.

prior to the Introduction of the Potentials.

43

certain circumstances, it was possible to render the electricity in some bodies more rare than it naturally is, and, by communicating this to other bodies, to give them an additional quantity, and make their electricity more dense."
In the same year in which Watson's theory was proposed, a certain Dr. Spence, who had lately arrived in America from Scotland, was showing in Boston some electrical experiments.
Among his audience was a man who already at forty years of
age was recognized as one of the leading citizens of the English

colonies in America, Benjamin Franklin of Philadelphia (b. 1706,

d. 1790).

Spence's experiments

"
were,"

writes

Franklin,*

"
imperfectly

performed,

as

he

was not

very expert ;

but, being

on a subject quite new to me, they equally surprised and

pleased me." Soon after this, the "Library Company" of

Philadelphia (an institution founded by Franklin himself) received from Mr. Peter Collinson of London a present of a glass tube, with some account of its use. In a letter written to

Collinson on July llth, 1747,f Franklin described experiments
made with this tube, and certain deductions which he had drawn from them.

If one person A, standing on wax so that electricity cannot
pass from him to the ground, rubs the tube, and if another
person B, likewise standing on wax, passes his knuckle along
A near the glass so as to receive its electricity, then both and B
will be capable of giving a spark to a third person C standing
A on the floor; that is, they will be electrified. If, however,
and B touch each other, either during or after the rubbing, they
will not be electrified.

This observation suggested to Franklin the same hypothesis

that (unknown to him) had been propounded a few months

previously by Watson : namely, that electricity is an element

present in a certain proportion in all matter in its normal

condition ;

so that,

before

the

rubbing, each of

the

persons

A,

B, and C has an equal share. The effect of the rubbing is to

* Franklin's Autobiography.
t Franklin's New Experiments and Observations on Electricity, letter ii.

44

Electric and Magnetic Science

transfer some of A's electricity to the glass, whence it is
A transferred to B. Thus has a deficiency and B a superfluity

of electricity ; and if either of them approaches C, who has the

normal amount, the distribution will be equalized by a spark.

A If, however,

B and

are in contact, electricity flows between

them so as to re-establish the original equality, and neither is

then electrified with reference to C.

Thus electricity is not created by rubbing the glass, but only transferred to the glass from the rubber, so that the
rubber loses exactly as much as the glass gains ; the, total
quantity of electricity in any insulated system is invariable. This assertion is usually known as the principle of conservation of

electric charge.

A The condition of and B in the experiment can evidently

A be expressed by plus and minus signs : having a deficiency

B - e and

a superfluity + e of electricity.

Franklin, at the

commencement of his own experiments, was not acquainted

with

du

Fay's

discoveries ;

but

it

is

evident

that

the

electric

fluid of Franklin is identical with the vitreous electricity of

du Fay, and that du Fay's resinous electricity is, in Franklin's

theory, merely the deficiency of a stock of vitreous electricity supposed to be possessed naturally by all ponderable bodies.
In Franklin's theory we are spared the necessity for admitting that two quasi-material bodies can by their union annihilate each other, as vitreous and resinous electricity were supposed to do.
Some curiosity will naturally be felt as to the considerations which induced Franklin to attribute the positive character to vitreous rather than to resinous electricity. They seem to have been founded on a comparison of the brush discharges from conductors charged with the two electricities; when the electricity was resinous, the discharge was observed to spread over the surface of the opposite conductor " as if it flowed from it." Again, if a Leyden jar whose inner coating is electrified vitreously is discharged silently by a conductor, of whose pointed ends one is near the knob and the other near the outer coating, the point which is near the knob is seen in the dark to be illumi-

prior to the Introduction of the Potentials.

45

nated with a star or globule, while the point which is near the outer coating is illuminated with a pencil of rays; which suggested to Franklin that the electric fluid, going from the inside to the outside of the jar, enters at the former point and
issues from the latter. And yet again, in some cases the flame of a wax taper is blown away from a brass ball which is
discharging vitreous electricity, and towards one which is discharging resinous electricity. But Franklin remarks that the interpretation of these observations is somewhat conjectural, and that whether vitreous or resinous electricity is the actual electric fluid is not certainly known.

Regarding the physical nature of electricity, Franklin held

much the same ideas as his contemporaries ; he pictured it as

an

elastic*

fluid,

consisting

of

" particles

extremely

subtile, since

it can permeate common matter, even the densest metals, with

such ease and freedom as not to receive any perceptible
resistance." He departed, however, to some extent from the

conceptions of his predecessors, who were accustomed to ascribe

all electrical repulsions to the diffusion of effluvia from the

excited

electric

to

the

body

acted on ;

so that

the

tickling

sensation which is experienced when a charged body is brought

near to the human face was attributed to a direct action of the

effluvia on the skin. This doctrine, which, as we shall see,

practically ended with Franklin, bears a suggestive resemblance to that which nearly a century later was introduced by

Faraday ; both explained electrical phenomena without introducing action at a distance, by supposing that something which forms an essential part of the electrified system is present at the spot where any electric action takes place ; but in the older theory this something was identified with the electric fluid itself, while in the modern view it is identified with a state of

stress in the aether. In the interval between the fall of one

school and the rise of the other, the theory of action at a distance was dominant.

The germs of the last-mentioned theory may be found in

*i.c., repulsive of its own particles.

46

Electric and Magnetic Science

Franklin's own writings. It originated in connexion with the

explanation of the Leyden jar, a matter which is discussed

in his third letter to Collinson, of date September 1st, 1747.

In charging the jar, he says, a quantity of electricity is taken

away from one side of the glass, by means of the coating

in contact with it, and an equal quantity is communi-

cated to the other side, by means of the other coating. The

glass itself he supposes to be impermeable to the electric

fluid, so that the deficiency on the one side can permanently

coexist with the redundancy on the other, so long as the two

sides are not connected

with

each other ;

but

when a con-

nexion is set up, the distribution of fluid is equalized through
the body of the experimenter, who receives a shock.

Compelled by this theory of the jar to regard glass as impenetrable to electric effluvia, Franklin was nevertheless well

aware* that the interposition of a glass plate between an electrified body and the objects of its attraction does not shield
the latter from the attractive influence. He was thus driven to

supposef that the surface of the glass which is nearest the excited body is directly affected, and is able to exert an

influence through the

glass

on

the opposite

surface ;

the latter

surface, which thus receives a kind of secondary or derived

excitement, is responsible for the electric effects beyond it.

This idea harmonized admirably with the phenomena of the jar ; for it was now possible to hold that the excess of

electricity on the inner face exercises a repellent action through

the substance of the glass, and so causes a deficiency on the

outer faces by driving away the electricity from it.J

Franklin had thus arrived at what was really a theory of

action at a distance between

the particles of

the

electric

fluid ;

and this he was able to support by other experiments.

"
Thus,"

he writes, " the stream of a fountain, naturally dense and con-

tinual, when electrified, will separate and spread in the form of

a brush, every drop endeavouring to recede from every other

* New Experiments, 1750, 28.
J Ibid., 1750, 32.

t Hid., 1750, 34. Letter v.

prior to the Introduction of the Potentials.

47

drop.' In order to account for the attraction between

oppositely charged bodies, in one of which there is an excess of

electricity as compared with ordinary matter, and in the other

an excess of ordinary matter as compared with electricity, he

assumed

that

"
though

the

particles

of

electrical

matter

do

repel

each other, they are strongly attracted by all other matter " ; so

that " common matter is as a kind of spunge to the electrical

fluid."

These repellent and attractive powers he assigned only to the actual (vitreous) electric fluid; and when later on the
mutual repidsion of resinously electrified bodies became known to him,* it caused him considerable perplexity.f As we shall see, the difficulty was eventually removed by.Aepinus.

In spite of his belief in the power of electricity to act at a distance, Franklin did not abandon the doctrine of effluvia.

"The form of the electrical atmosphere," he says,} "is that of the

body it surrounds. This shape may be rendered visible in a still
air, by raising a smoke from dry rosin dropt into a hot tea-

spoon under the electrified body, which will be attracted, and spread itself equally on all sides, covering and concealing the
body, And this form it takes, because it is attracted by all

parts of the surface of the body, though it cannot enter the substance already replete. Without this attraction, it would
not remain round the body, but dissipate in the air." He

observed, however, that electrical effluvia do not seem to

affect, or be affected by,

the air ;

since

it

is

possible to breathe

freely in the neighbourhood of electrified bodies ; and moreover a current of dry air does not destroy electric attractions and

repulsions.
Kegarding the suspected identity of electricity with the matter of heat, as to which Nollet had taken the affirmative
position, Franklin expressed no opinion. " Common fire," he

* He refers to it in his Paper read to the Royal Society, December 18, 1755.
t Cf. letters xxxvii and xxxviii, dated 1761 and 1762.
1 New Experiment* , 1750, 15.
Letter vii, 1751.

48

Electric and Magnetic Science

writes,*

" is

in

all

bodies, more

or

less, as well as

electrical fire.

Perhaps they may be different modifications of the same

element ;

or they may be

different

elements.

The latter is by

some suspected. If they are different things, yet they may and
do subsist together in the same body."

Franklin's work did not at first receive from European

philosophers

the

attention

which

it

deserved ;

although

Watson

generously endeavoured to make the colonial writer's merits

known,f and inserted some of Franklin's letters in one of his own

papers communicated to the Eoyal Society. But an account of

Franklin's discoveries, which had been printed in England, happened to fall into the hands of the naturalist Buffon, who was so much impressed that he secured the issue of a French transla-

tion of the work ;

and it was this publication which, as we have

seen, gave such offence to Nollet. The success of a plan proposed

by Franklin for drawing lightning from the clouds soon engaged

public attention everywhere; and in a short time the triumph

of the one-fluid theory of electricity, as the hypothesis of

Watson and Franklin is generally called, was complete. Collet,

who was obdurate, "lived to see himself the last of his sect,

except Monsieur B

of Paris, his eleve and immediate

disciple." J
The theory of effluvia was finally overthrown, and replaced

by that of action at a distance, by the labours of one of

Franklin's continental followers, Francis Ulrich Theodore

Aepinus (&. 1724, d. 1802). The doctrine that glass is impermeable to electricity, which had formed the basis of

Franklin's theory of the Leyden phial, was generalized by Aepinus|| and his co-worker Johann Karl Wilcke (5. 1732, d. 1796)

into the law that all non-conductors are impermeable to the

* Letter v.

Cx_J

tPhil. Trans, xlvii, p. 202.

Watson agreed with Nollet in rejecting Franklin's

theory of the impermeability of glass.

J Franklin's Autobiography.

This philosopher's surname had been hellenized from its original form Hoeck

to alveivos by one of his ancestors, a distinguished theologian.

F. V.
||

T.

Aepinus

Tentamen

Thcoriae

Elcctricitatis

et

Magnetismi :

St. Petersburg, 1759.

prior to the Introduction of the Potentials.

49

electric fluid. That this applies even to air they proved by

constructing a machine analogous to the Leyden jar, in which,

however, air took the place of glass as the medium between

two oppositely charged surfaces. The success of this experi-

ment led Aepinus to deny altogether the existence of electric

effluvia surrounding charged bodies :* a position which he

regarded as strengthened by Franklin's observation, that the electric field in the neighbourhood of an excited body is not destroyed when the adjacent air is blown away. The electric

fluid must therefore be supposed not to extend beyond the excited bodies themselves. The experiment of Gray, to which

we have already referred, showed that it does not penetrate far into their substance; and thus it became necessary to

suppose that the electric fluid, in its state of rest, is con-

fined to thin layers on the surfaces of the excited bodies.

This being granted, the attractions and repulsions observed

between the bodies compel us to believe that electricity acts

at a distance across the intervening air.

Since two vitreously charged bodies repel each other, the

force between two particles of the electric fluid must (on

Franklin's one-fluid theory, which Aepinus adopted) be

repulsive : and since there is 'an attraction between oppositely

charged bodies, the force between electricity and ordinary

matter must be attractive. These assumptions had been made,

as we have seen, by Franklin; but in order to account for

the repulsion between two resinously charged bodies, Aepinus introduced a new supposition namely, that the particles

of ordinary matter repel each other. This, at first, startled

his contemporaries; but, as he pointed out, the "unelectrified"

matter with which we are acquainted is really matter saturated

with its natural quantity of the electric fluid, and the forces

due to the matter and fluid balance each

other ;

or perhaps,

as he suggested, a slight want of equality between these

forces might give, as a residual, the force of gravitation. Assuming that the attractive and repellent forces increase as "

* This was also maint.iined about the same time by Giacomo Battista Beet-aria
of Turin (b. 1716, d. 1781;.
E

50

Electric and Mag<nvetic Science

the distance between the acting charges decreases, Aepinus

applied his theory to explain a phenomenon which had been

more or less indefinitely observed by many previous writers, and

specially studied a short time previously by John Canton*

(&. 1718, d. 1772) and by Wilckef namely, that if a conductor is brought into the neighbourhood of an excited body without

actually touching it, the remoter portion of the conductor

acquires an electric charge of the same kind as that of the

excited body, while the nearer portion acquires a charge of the
opposite kind. This effect, which is known as the induction of

electric charges, had been explained by Canton himself and by

Franklin} in terms of the theory of electric effluvia. Aepinus

showed that it followed naturally from the theory of action at a

distance, by taking into account the mobility of the electric fluid

in

conductors ;

and

by discussing

different

cases,

so

far as was

possible with the means at his command, he laid the foundations

of the mathematical theory of electrostatics.

Aepinus did not succeed in determining the law according to

which the force between two electric charges varies with the

distance between

them ;

and

the

honour of having first accom-

plished this belongs to Joseph Priestley (b. 1733, d. 1804), the
discoverer of oxygen. Priestley, who was a friend of Franklin's,

had been informed by the latter that he had found cork balls to

be wholly unaffected by the electricity of a metal cup within

which they were held ; and Franklin desired Priestley to repeat

and ascertain the fact. Accordingly, on December 21st, 1766,

Priestley instituted experiments, which showed that, when a

hollow metallic vessel is electrified, there is no charge on the inner

surface (except near the opening), and no electric force in the air
inside. From this he at once drew the correct conclusion, which was published in 1767. " May we not infer," he says, "from

*Phil. Trans, xlviii (1753), p. 350.
t Disputatio physica experimentalis de electricitatibus contrariis : Rostock, 1757. J In liis paper read to the Royal Society on Dec. 18th, 1755.
J. Priestley, The History and Present State of Electricity, with Original Experiments ; London, 1767: page 732. That electrical attraction follows the law of the inverse square had been suspected -by Daniel Bernoulli in 1760: Cf.
Sochi's Experiments, Ada Helvetica, iv, p. 214.

prior to the Introduction of the Potentials.

51

this experiment that the attraction of electricity is subject to

the same laws with that of gravitation, and is therefore according

to the squares of

the

distances ;

since it is easily demonstrated

that were the earth in the form of a shell, a body in the inside

of

it

would

not

be

attracted

to

one

side

more

than

another

"
?

This brilliant inference seems to have been insufficiently
studied by the scientific men of the day ; and, indeed, its author
appears to have hesitated to claim for it the authority of a com-
plete and rigorous proof. Accordingly we find that the question of the law of force was not regarded as finally settled for eighteen
years afterwards.*
By Franklin's law of the conservation of electric charge, and
Priestley's law of attraction between charged bodies, electricity was raised to the position of an exact science. It is impossible to mention the names of these two friends in such a connexion

without reflecting on the curious parallelism of their lives. In
both men there was the same combination of intellectual bold-

ness and power with moral earnestness and public spirit. Both

.of them carried on a long and tenacious struggle with the reac-

tionary influences which dominated the English Government in

.the reign of George

III ;

and

both at last, when overpowered in

the conflict, reluctantly exchanged their native flag for that of

the United States of America. The names of both have been

held in honour by later generations, not more for their scientific discoveries than for their services to the cause of

religious, intellectual, and political freedom.

The most celebrated electrician of Priestley's contemporaries

in London was the Hon. Henry Cavendish (b. 1731, d. 1810),

whose interest in the subject was indeed hereditary, for his

father, Lord Charles Cavendish, had assisted in Watson's experi-

ments of 1747.f In 1771 Cavendish} presented to the Koyal

Society

an

"
Attempt

to

explain

some

of

the

principal phenomena

of Electricity, by means of an elastic fluid." The hypothesis j

* In 1769 Dr. John Robison (b. 1739, d. 1805), of Edinburgh, endeavoured to

determine the law of force by direct experiment, and found it to be tbat of the

inverse 2'06th power of the distance.

t Phil. Trans, xlv, p. 67 (1750).

J Phil. Trans. Ixi, p. 584 (1771).

E2

52

Electric and Magnetic Science

adopted is that of the one-fluid theory, in much the same form

as that of Aepinus. It was, as he tells us, discovered indepen-

dently, although he became acquainted with Aepinus' work

before the publication of his own paper.

In this memoir Cavendish makes no assumption regarding

the law of force between electric charges, except that it is

" inversely as some less power of the distance

than

the

"
cube ;

but he evidently inclines to believe in the law of the inverse square. Indeed, he shows it to be " likely, that if the electric

attraction or repulsion is inversely as the square of the distance,

almost all the redundant fluid in the body will be lodged close to the surface, and there pressed close together, and the rest of the body will be saturated"; which approximates closely to the discovery made four years previously by Priestley. Cavendish

did, as a matter of fact, rediscover the inverse square law shortly afterwards; but, indifferent to fame, he neglected to communicate
to others this and much other work of importance. The value of

his researches was not realized until the middle of the nineteenth

century, when William Thomson (Lord Kelvin) found in Caven-

dish's manuscripts the correct value for the ratio of the electric charges carried by a circular disk and a sphere of the same radius
which had been placed in metallic connexion. Thomson urged that the papers should be published ; which came to pass* in

1879, a hundred years from the date of the great discoveries which they enshrined. It was then seen that Cavendish had

anticipated his successors in several of the ideas which will

presently be discussed amongst others, those of electrostatic capacity and specific inductive capacity.
In the published memoir of 1771 Cavendish worked out the consequences of his fundamental hypothesis more completely

than Aepinus ; and, in fact, virtually introduced the notion of

electric potential, though, in the absence of any definite assumption as to the law of force, it was impossible to develop this idea

to any great extent.

* The Electrical Researches of the Hon. Henry Cavendish, edited by J. Clerk Maxwell, 1879.

prior to the Introduction of the Potentials.

53

One of the investigations with which Cavendish occupied himself was a comparison between the conducting powers of different materials for electrostatic discharges. The question had been first raised by Beccaria, who had shown* in 1753 that when the circuit through which a discharge is passed contains tubes of water, the shock is more powerful when the cross-section of the tubes is increased. Cavendish went into the matter
much more thoroughly, and was able, in a memoir presented to
the Eoyal Society in 1775,f to say : " It appears from some experiments, of which I propose shortly to lay an account before this Society, that iron wire conducts about 400 million times better than rain or distilled water that is, the electricity meets with no more resistance in passing through a piece of iron wire 400,000,000 inches long than through a column of water of the same diameter only one inch long. Sea- water, or a solution of one part of salt in 30 of water, conducts 100 times, or a saturated
solution of sea-salt about 720 times, better than rain-water."
The promised account of the experiments was published in the volume edited in 1879. It appears from it that the method of testing by which Cavendish obtained these, results was simply that of physiological sensation; but the figures given in the comparison of iron and sea-water are remarkably exact.
While the theory of electricity was being established on a sure foundation by the great investigators of the eighteenth century, a no less remarkable development was taking place in the kindred science of magnetism, to which our attention must now be directed.
The law of attraction between magnets was investigated at an earlier date than the corresponding law for electrically charged bodies. Newton, in the Principia says : " The power of gravity is of a different nature from the power of magnetism. For the magnetic attraction is not as the matter attracted. Some bodies are attracted more by the magnet, others less ; most bodies not at all. The power of magnetism, in one and the same

DdV * G. B. Beccaria,

ehttridsmo artificiale e natural*, Turin. 1753, p. 113.

+ Phil. Trans. Ixvi (1776), p. 196.

% Book iii, Prop, vi, cor. 5.

54

Electric and Magnetic Science

body, may

be

increased and

diminished and ;

is

sometimes

far

stronger, for the quantity of matter, than the power of gravity ;

and in receding from the magnet, decreases not in the duplicate,

but almost in the triplicate proportion of the distance, as nearly

as I could judge from some rude observations."

The edition of ihePrincipia which was published in 1742 by

Thomas Le Seur and Francis Jacquier contains a note on this

corollary, in which the correct result is obtained that the

directive couple exercised on one magnet by another is

proportional to the inverse cube of the distance.

The

first

discoverer

of

the

law

of

force

between

1
magnetic

\ poles was John Michell (b. 1724, d. 1793), at that time a young
Fellow of Queen's College, Cambridge,* who in 1750 published
A Treatise of Artificial Magnets ; in ivhich is shown an easy

and expeditious method of making them superior to the lest

natural ones. In this he states the principles of magnetic

theory as followsf :
" Wherever any Magnetism, is found, whether in the Magnet

itself, or any piece of Iron, etc., excited by the Magnet, there are

always found two Poles, which are generally called North and

South ;

and the North Pole

of

one

Magnet

always attracts

the

South Pole, and repels the North Pole of another: and wee versa"

This is of course adopted from Gilbert.

"Each Pole attracts or repels exactly equally, at equal

distances, in every direction." This, it may be observed, over-

throws the theory of vortices, with which it is irreconcilable. " The Magnetical Attraction and Eepulsion are exactly equal to

each other." This, obvious though it may seem to us, was really a most important advance, for, as he remarks, " Most people, who

* Michell had taken his degree only two years previously. Later in life he was

on terms of

friendship with Priestley,

Cavendish, and

William

Herschel ;

it

was

he who taught Herschel the art of grinding mirrors for telescopes. The plan of

determining the density of the earth, which was carried out by Cavendish in 1798, and is generally known as the " Cavendish Experiment," was due to Michell.

Michell

was

the

first

inventor

of

the torsion-balance ;

he

also

made

many

valuable

contributions to Astronomy. In 1767 he became Rector of Thornhill, Yorks,

and lived there until his death.

t Loc. cit., p. 17.

-^

prior to the Introduction of the Potentials.

55

have mention'd any thing relating to this property of the Magnet, have agreed, not only that the Attraction and Repulsion of Magnets are not equal to each other, but that also, they do not observe the same rule of increase and decrease."
" The Attraction and Eepulsion of Magnets decreases, as the Squares of the distances from the respective poles increase." This great discovery, which is the basis of the mathematical theory of Magnetism, was deduced partly from his own observations, and partly from those of previous investigators (e.g. Dr. Brook Taylor and P. Muschenbroek), who, as he observes, had made accurate experiments, but had failed to take into account all the considerations necessary for a sound theoretical
discussion of them.

After Michell the law of the inverse square was maintained

by Tobias Mayer* of Gottingen (&. 1723, d. 1762), better known

as the

author of

Lunar Tables which were long in

use ;

and by

the celebrated mathematician, Johann Heinrich Lambertf (b.

1728, d. 1777).
The promulgation of the one-fluid theory of electricity, in the middle of the eighteenth century, naturally led to attempts to construct a similar theory of magnetism ; this was effected in 1759 by AepinusJ, who supposed the "poles "to be places at which a magnetic fluid was present in amount exceeding or falling short of the normal quantity. The permanence of magnets was accounted for by supposing the fluid to be entangled in their pores, so as to be with difficulty displaced. The particles of the fluid were assumed to repel each other, and to attract the particles of iron and steel ; but, as Aepinus saw, in order to satisfactorily explain magnetic phenomena it was necessary to assume also a mutual repulsion among the material particles of the

magnet. Subsequently two imponderable magnetic fluids, to which

* Noticed in Gottinger Gelehrter Anzeiger, 1760 : cf. Aepinus, Nov. Comm. Acad. Petrop., 1768, and Mayer's Opera Inedita, herausg. von G. C. Lichtenberg.
\-Histoirede V Acad. de Berlin, 1766, pp. 22, 49. % In the Tentamen, to which reference has already been made.

56

Electric and Magnetic Science

the names boreal and austral were assigned, were postulated by the Hollander Anton Brugmans (5. 1732, d. 1789) and by Wilcke. These fluids were supposed to have properties of mutual attraction and repulsion similar to those possessed by vitreous and resinous electricity.
The writer who next claims our attention for his services

both to magnetism and to electricity is the French physicist,
Charles Augustin Coulomb* (ft. 1736, d. 1806). By aid of the
torsion-balance, which was independently invented by Michell and himself, he verified in 1785 Priestley's fundamental law that the repulsive force between two small globes charged with the same kind of electricity is in the inverse ratio of the square of the distance of their centres. In the second memoir he

extended this law to the attraction of opposite electricities.

Coulomb did not accept the one-fluid theory of Franklin,

Aepinus, and Cavendish, but preferred a rival hypothesis which

had been proposed in 1759 by Kobert Symmer.f

My " notion,"

said Symmer, " is that the operations of electricity do not depend

upon one single positive power, according to the opinion generally

received; but upon two distinct, positive, and active powers,

which, by contrasting, and, as it were, counteracting each other,

produce the various phenomena of electricity ; and that, when a

body is said to be positively electrified, it is not simply that it is

possessed of a larger share of electric matter than in a natural

state ;

nor, when it is

said

to be

negatively

electrified,

of a

less ;

but that, in the former case, it is possessed of a larger portion

of one of those active powers, and in the latter, of a larger

portion

of

the

other ;

while

a

body

in

its

natural

state

remains

unelectrified, from an equal ballance of those two powers within

it."
Coulomb developed this idea : " Whatever be the cause of electricity," he says,J " we can explain all the phenomena by

* Coulomb's First, Second, and Third Memoirs appear in Memoires de 1'Acad.,

1785 ;

the Fourth in 1786, the Fifth

in 1787, the Sixth in

1788,

and the Seventh

in 1789.

t Phil. Trim*, li (1759), p. 371.

j Sixth Memoir, p. 561.

prior to the Introduction of the Potentials.

57

supposing that there are two electric fluids, the parts of the

same fluid repelling each other according to the inverse square

of the distance, and attracting the parts of the other fluid

according to the same inverse square law."

" The

^
supposition

of two fluids," he adds, " is moreover in accord with all those 7

discoveries of modern chemists and physicists, which have made

known to us various pairs of gases whose elasticity is destroyed

by their admixture in certain proportions an effect which could

not take place without something equivalent to a repulsion

between the parts of the same gas, which is the cause of its

elasticity, and an attraction between the parts of different

gases, which accounts for the loss of elasticity on combination." J

According,

then,

to

the

two-fluid

theory,

the

"

natural

" fluid

contained in all matter can be decomposed, under the influence

of an electric field, into equal quantities of vitreous and

resinous electricity, which, if the matter be conducting, can then

fly to the surface of the body. The abeyance of the characteristic properties of the opposite electricities when in combination was
f
sometimes further compared to the neutrality manifested by .

the compound of an acid and an alkali.

The publication of Coulomb's views led to some controversy

between

the

partisans

of

the

one-fluid

and

two-fluid

theories ;

the

latter was soon generally adopted in France, but was stoutly

opposed in Holland by Van Marum and in Italy by Volta.

The chief difference

between the rival hypotheses

is

that,

in

^
the

two-fluid theory, both the electric fluids are movable within the

substance

of

a

solid

conductor ;

while

in

the

one-fluid theory

the

actual electric fluid is mobile, but the particles of the conductor

are fixed. The dispute could therefore be settled only by a determination of the actual motion of electricity in discharges ; and this was beyond the reach of experiment.

In his Fourth Memoir Coulomb showed that electricity in

equilibrium is confined to the surface of conductors, and does

not penetrate

to their

interior

substance ;

and in

the

Sixth

Memoir* he virtually establishes the result that the electric

* Page 677.

58

Electric and Magnetic Science

force near a conductor is proportional to the surface-density of
electrification.

Since the overthrow of the doctrine of electric effluvia by Aepinus, the aim of electricians had been to establish their
science upon the foundation of a law of action at a distance, resembling that which had led to such triumphs in Celestial
Mechanics. When the law first stated by Priestley was at
length decisively established by Coulomb, its simplicity and beauty gave rise to a general feeling of complete trust in it as the best attainable conception of electrostatic phenomena. The result was that attention was almost exclusively focused on action-at-a-distance theories, until the time, long afterwards,, when Faraday led natural philosophers back to the right'

path.
Coulomb rendered great services to magnetic theory. It was he who in 1777, by simple mechanical reasoning, completed
the overthrow of the hypothesis of vortices.* He also, in the
second of the Memoirs already quoted,f confirmed Michell's law, according to which the particles of the magnetic fluids attract or repel each other with forces proportional to the inverse square of the distance. Coulomb, however, went beyond this, and endeavoured to account for the fact that the two
magnetic fluids, unlike the two electric fluids, cannot be obtained separately; for when a magnet is broken into two pieces, one containing its north and the other its south pole, it is found that each piece is an independent magnet possessing two poles of its own, so that it is impossible to obtain a north or south pole in a state of isolation. Coulomb explained this by supposing^ that the magnetic fluids are permanently imprisoned within the molecules of magnetic bodies, so as to be incapable of crossing from one molecule to the next each molecule therefore under all
;
circumstances contains as much of the boreal as of the

* Mem. presences par divers Savans, ix (1780), p. 165.

Mem t

de 1'Acad., 1785, p. 593. Gauss finally established the law by a

much more refined method.

J In his Seventh Memoir, Mem, de 1'Acad., 1789, p. 488.

prior to the Introduction of the Potentials.

59

austral fluid, and magnetization consists simply in a separation of the two fluids to opposite ends of each molecule. Such

a hypothesis evidently accounts for the impossibility of separating the two fluids to opposite ends of a body of finite
size. The same idea, here introduced for the first time, has

since been applied with success in other departments of

electrical philosophy.
In spite of the advances which have been recounted, the mathematical development of electric and magnetic theory was scarcely begun at the close of the eighteenth century ; and
many erroneous notions were still widely entertained. In a

Eeport* which was presented to the French Academy in 1800, it was assumed that the mutual repulsion of the particles of electricity on the surface of a body is balanced by the resistance of the surrounding air; and for long afterwards

the electric force outside a charged conductor was confused

with a supposed additional pressure in the atmosphere.

Electrostatical theory was, however, suddenly advanced to

quite a mature state of development by Simeon Denis Poisson (b. 1781, d. 1840), in a memoir which was read to the French Academy in 1812.f As the opening sentences show, he accepted

the conceptions of the two-fluid theory.

" The theory of electricity which is most generally accepted,"

he says,

" is

that which

attributes

the

phenomena

to two

different fluids, which are contained in all material bodies.

It is supposed that molecules of the same fluid repel each other and attract the molecules of the other fluid these
;

forces of attraction and repulsion obey the law of the inverse

square

of

the distance ;

and at the same

distance

the attractive

power is equal to the repellent power; whence it follows

that, when all the parts of a body contain equal quantities

of the two fluids, the latter do not exert any influence on

the fluids contained in neighbouring bodies, and consequently no electrical effects are discernible. This equal and uniform

* On Yolla's discoveries. t Mem. de Plnstitut, 1811, Part i., p. 1, Part ii., p. 163.

60

Electric and Magnetic Science

distribution of the two fluids is called the natural state when this ;

state is disturbed in any body, the body is said to be electrified,

and the various phenomena of electricity begin to take place.

"Material bodies do not all behave in the same way with

respect to the electric fluid : some, such as the metals, do

not appear to exert any influence on it, but permit it to

move

about freely

in their

substance ;

for this reason

they

are called conductors. Others, on the contrary very dry air,

for example oppose the passage of the electric fluid in their

interior, so that they can prevent the fluid accumulated in

conductors from being dissipated throughout space."
When an excess of one of the electric fluids is communi-

cated to a metallic body, this charge distributes itself over the

surface of the body, forming a layer whose thickness at any

point depends on the shape of the surface. The resultant force

due to the repulsion of all the particles of this surface-layer

must vanish at any point in the interior of the conductor, since

otherwise the natural state

existing there

would

be

disturbed ;

and Poisson showed that by aid of this principle it is possible

in certain cases to determine the distribution of electricity in

the surface-layer. For example, a well-known proposition of

the theory of Attractions asserts that a hollow shell whose

bounding surfaces are two similar and similarly situated

ellipsoids exercises 110 attractive force at any point within the

interior hollow; and it may thence be inferred that, if an

electrified metallic conductor has the form of an ellipsoid, the

charge will be distributed on it proportionally to the normal

distance from the surface to an adjacent similar and similarly

situated ellipsoid.

Poisson went on to show that this result was by no means all

that might with advantage be borrowed from the theory of

I Attractions. Lagrange, in a memoir on the motion of gravitating

bodies, had shown* that the components of the attractive force

* Mem. de Berlin, 1777. The theorem was afterwards published, and ascribed to Laplace, in a memoir by Legendre on the Attractions of Spheroids, which will be found in the Mem. par divers Snvanx, published in 178o.

prior to the Introduction of the Potentials.

61

at any point can be simply expressed as the derivates of the
function which is obtained by adding together the masses of all
the particles of an attracting system, each divided by its distance from the point; and Laplace had shown* that this
V function satisfies the equation

in space free from attracting matter. Poisson himself showed

later, in 1813,f that when the point (z, y, z) is within the

substance of the attracting body, this equation of Laplace must

be replaced by

W VV VV
^ + w~~vr:

p>

where p denotes the density of the attracting matter at the
point. In the present memoir Poisson called attention to the
F utility of this function in electrical investigations, remarking
that its value over the surface of any conductor must be

constant.

The known formulae for the attractions of spheroids show

that when a charged conductor is spheroidal, the repellent force

acting on a small charged body immediately outside it will be

directed at right angles to the surface of the spheroid, and will

be proportional to the thickness of the surface-layer of electricity

at this place. Poisson suspected that this theorem might be

true for conductors not having the spheroidal form a result

which, as we have seen, had been already virtually given by

Coulomb ;

and Laplace

suggested to Poisson the following

proof, applicable to the general case. The force at a point

immediately outside the conductor can be divided into a

part s due to the part of the charged surface immediately
adjacent to the point, and a part S due to the rest of

the surface. At a point close to this, but just inside the con-

ductor, the force j^jpll still act; but the forces will evidently

* Mem. de 1'Acad., 1782 (published in 1785), p. 113.
t Bull, de la Soc. Philomathique. iii. (1813,, p. 388.

62

Electric and Magnetic Science

be reversed in direction. Since the resultant force at the latter

point vanishes, we must have S=s ; so the resultant force at the
exterior point is 2s. But s is proportional to the charge per

unit area of the surface, as is seen by considering the case of

an infinite plate ; which establishes the theorem.
When several conductors are in presence of each other, the

distribution of electricity on their surfaces may be determined

by the principle, which Poisson took as the basis of his work, that at any point in the interior of any one of the conductors, the resultant force due to all the surface -layers must be zero.
He discussed, in particular, one of the classical problems of

electrostatics namely, that of determining the surface-density

on two charged conducting spheres placed at any distance from

each other. The solution depends on Double Gamma Functions

in

the

general

case ;

when

the

two

spheres

are

in

contact,

it

depends on ordinary Gamma Functions. Poisson gave a solution

in terms of definite integrals, which is equivalent to that in

terms of Gamma Functions ; and after reducing his results to

numbers, compared them with Coulomb's experiments.

f

The rapidity with which in a single memoir Poisson passed

from the barest elements of the subject to such recondite

problems as those just mentioned may well excite admiration.
His success is, no doubt, partly explained by the high state of development to which analysis had been advanced by the great

mathematicians of the eighteenth century ; but even after
allowance has been made for what is due to his predecessors, Poisson' s investigation must be accounted a splendid memorial

u of his genius. Some years later Poisson turned his attention to magnetism ;

and, in a masterly paper* presented to the French Academy in

1824, gave a remarkably complete theory of the subject. His starting-point is Coulomb's doctrine of two imponderable

magnetic fluids, arising from the decomposition of a neutral fluid, and confined in their movements to the individual elements

* Mem. <le 1'Acad., v, p. 247.

prior to the Introduction of the Potentials.

63

of the magnetic body, so as to be incapable of passing from one element to the next
m Suppose that an amount of the positive magnetic fluid is
located at a point (x y, z) ; the components of the magnetic intensity, or force exerted on unit magnetic pole, at a point (, f, ) will evidently be
-m-f-X -m~(-\ -m-(-)

where r denotes

((?

-

xf

+

(n

-

2 ?/)

+

(Z

-

2
z) j*.

Hence if we

consider next a magnetic element in which equal quantities of

the two magnetic fluids are displaced from each other parallel

to_ the ic-axis, the components of the magnetic intensity at

(g, i|, 2) will be the negative derivates, with respect to ij,

respectively, of the function

where the quantity A, which does not involve (f, j, ), may be called the magnetic moment of the element : it may be measured

by the couple required to maintain the element in equilibrium

at a definite angular distance from the magnetic meridian.

If the displacement of the two fluids from each other in the

element

is

not

parallel

to

the

axis

of

x t

it

is

easily

seen

that

the

expression corresponding to the last is

where the vector (A, B, C) now denotes the magnetic moment
of the element.
Thus the magnetic intensity at an -external point (, 77, ) due to any magnetic body has the components

where

-

;

017

ex

oy

integrated throughout the substance of the magnetic body, and

64

Electric and Magnetic Science

where the vector (A, B, C) or I represents the magnetic moment
per unit-volume, or, as it is generally called, the magnetization.
The function Fwas afterwards named by Green the magnetic

potential.
Poisson, by integrating by parts the preceding expression for the magnetic potential, obtained it in the form

F = [[(I . dS). \ - fjp div I dx dy dz*

the first integral being taken over the surface $ of the magnetic

body, and the second integral being taken throughout its volume.

This formula shows that the magnetic intensity produced by the

body in external space is the same as would be produced by a

fictitious distribution of magnetic fluid, consisting of a layer

over

its

surface, of

surface-charge

(I .- dS)

per

element

dS y

together with a volume-distribution of density - div I through-

out its substance. These fictitious magnetizations are generally

known as Poisson's equivalent surface- and volume-distributions

of magnetism.

Poisson, moreover, perceived that at a point in a very small

cavity excavated within the magnetic body, the magnetic

potential has a limiting value which is independent of the shape

of the cavity as the

dimensions

of

the

cavity tend

to zero ;

but

that this is not true of the magnetic intensity, which in such a

small cavity depends on the shape of the cavity. Taking the

cavity to be spherical, he showed that the magnetic intensity

within it is

F grad

4
^-7rl,f

where I denotes the magnetization at the place.

* If the components of a vector a are denoted by (ax , ay , a z ), the quantity drbjc + a y by -f- at kz is called the scalar product of two vectors a and b, and is denoted

by (a . b). The quantity

^ ^ '
^+ +

fix

dy

02

is called the divergence of the vector a, and is

denoted by div a.

t The vector whose components are -
C

, - ?, - -

dy

dz

is denoted byJ grad V.

prior to the Introduction of the Potentials.

65

This memoir also contains a discussion of the magnetism temporarily induced in soft iron and other magnetizable metals

by the approach of a permanent magnet. Poisson accounted for

the properties of temporary magnets by assuming that they contain embedded in their substance a great number of small

spheres,

which

are

perfect

conductors

for

the

magnetic

fluids ;

so

that the resultant magnetic intensity in the interior of one of
these small spheres must be zero. He showed that such a sphere,

when placed in a field of magnetic intensity F,* must acquire a

magnetic moment of amount -.- F x the volume of the sphere,

in order to counteract within the sphere the force F. Thus if

kp denote the total volume of these spheres contained within a

unit volume of the temporary magnet, the magnetization will be

I, where

4-TrI = kp F,

and F denotes the magnetic intensity within a spherical cavity

excavated in the body. This is Poisson s laiv of induced magnetism.
It is known that some substances acquire a greater degree of temporary magnetization than others when placed in the

same circumstances : Poisson accounted for this by supposing that

the quantity kp varies from one substance to another. But the

experimental data show that for soft iron kp must have a value

very near unity, which would obviously be impossible if kp is to mean the ratio of the volume of spheres contained within a

region to the total volume of the region.f The physical inter-

pretation assigned by Poisson to his formulae must therefore be

rejected, although the formulae themselves retain their value.

Poisson's electrical and magiietical investigations were

generalized and extended in 1828 by George Green* (b. 1793,

d. 1841). Green's treatment is based on the properties of the

function already used by Lagrange, Laplace, and Poisson, which

* In the present work, vectors will generally be distinguished by heavy type.
t This objection was advanced by Maxwell in 430 of his Treatise. An attempt to overcome it was made by Betti : cf. p. 377 of his Lessons on the Potential.
J A.n essay on the application of mathematical analysis to the theories of electricity and magnetism, Nottingham, 1828 : reprinted in The Mathematical Papers ofthe late
George Green, p. 1.
F

66

Electric and Magnetic Science.

represents the sum of all the electric or magnetic charges in the field, divided by their respective distances from some given point : to this function Green gave the name potential, by which it has

always since been known.* Near the beginning of the memoir is established the
celebrated formula connecting surface and volume integrals,
which is now generally called G-reeris Theorem, and of which
Poisson's result on the equivalent surface- and volume-distribu-

tions of magnetization is a particular application. By using

this theorem to investigate the properties of the potential,

Green arrived at many results of remarkable beauty and

We interest.

need only mention, as an example of the power

of his method, the following : Suppose that there is a hollow

conducting shell, bounded by two closed surfaces, and that a number of electrified bodies are placed, some within and some

without it ; and let the inner surface and interior bodies be

called the interior system, and the outer surface and exterior botlies be called the exterior system. Then all the electrical

phenomena of the interior system, relative to attractions, repulsions, and densities, will be the same as if there were no

exterior system, and the inner surface were a perfect conductor,

put in communication with the

earth ;

and

all

those of

the

exterior system will be the same as if the interior system did not

exist, and the outer surface were a perfect conductor, containing

a quantity of electricity equal to the whole of that originally contained in the shell itself and in all the interior bodies.

It will be evident that electrostatics had by this time attained a state of development in which further progress could be hoped for only in the mathematical superstructure, unless experiment should unexpectedly bring to light phenomena of an entirely new character. This will therefore be a convenient place to pause and consider the rise of another branch of

electrical philosophy.

* Euler in 1744 (De melhodis inveniendi . . .) had spoken of the vis potentialis what would now be called the potential energy possessed by an elastic body when bent.

CHAPTEE III.

GALVANISM, FROM GALVANI TO OHM.

UNTIL the last decade of the eighteenth century, electricians were occupied solely with statical electricity. Their attention was then turned in a different direction.

In a work entitled Recherches sur Vorigine des sentiments agreables et cUsagrcables, which was published* in 1752, Johann Georg Sulzer (b. 1720, d. 1779) had mentioned that, if two pieces of metal, the one of lead and the other of silver, be joined together in such a manner that their edges touch, and if they be placed on the tongue, a taste is perceived " similar to
that of vitriol of iron," although neither of these metals applied separately gives any trace of such a taste. " It is not probable," he says, " that this contact of the two metals causes a solution of either of them, liberating particles which might affect the
tongue : and we must therefore conclude that the contact sets up a vibration in their particles, which, by affecting the nerves
of the tongue, produces the taste in question." This observation was not suspected to have any connexion
with electrical phenomena, and it played no part in the inception of the next discovery, which indeed was suggested by a mere accident.

Luigi Galvani, born at Bologna in 1737, occupied from 1775
onwards a chair of Anatomy in his native city. For many years before the event which made him famous he had been studying
the susceptibility of -the nerves to irritation ; and, having been <-
formerly a pupil of Beccaria, he was also interested in electrical experiments. One day in the latter part of the year 1780 he ' had, as he tells us,f " dissected and prepared a frog, and laid it on a table, on which, at some distance from the frog, was an
electric machine. It happened by chance that one of my

* Mem. de 1'Acad. de Berlin, 1752, p. 356.
E t Aloysii Galvani, De Viribus 'lee trie itatis in Motu Mnsculari : Commentarii

Bononiensi, vii (1791), p. 363.

F2

68

Galvanism, from Galvani to Ohm.

assistants touched the inner crural nerve of the frog with the

point of a scalpel ; whereupon at once the muscles of the limbs

were violently convulsed.
" Another of those who used to help me in electrical experi-

ments thought he had noticed that at this instant a spark was

drawn from the conductor of the machine. I myself was at the

time occupied with a totally different matter; but when he

my drew

attention to this, I greatly desired to try it for myself,.

and discover its hidden principle. So I, too, touched one or

other of the crural nerves with the point of the scalpel, at the

same time that one of those present drew a spark ; and the same

phenomenon was repeated exactly as before."*

After this, Galvani conceived the idea of trying whether the

electricity of thunderstorms would induce muscular contractions

equally well with the electricity of the machine. Having

successfully

experimented

with

lightning,

he

" wished,"

as

he

writes,! " to try the effect of atmospheric electricity in calm

My weather.

reason for this was an observation I had made,,

that frogs which had been suitably prepared for these experi-

ments and fastened, by brass hooks in the spinal marrow, to

the

iron

lattice

round a

certain

hanging-garden

my at

house,,

exhibited convulsions not only during thunderstorms, but

sometimes even when the sky was quite serene. I suspected

these effects to be due to the changes which take place during

the day in the electric state of the atmosphere ; and so, with

some degree of confidence, I performed experiments to test the

point; and at different hours for many days I watched frogs

which I had disposed for the purpose ; but could not detect any

motion in their muscles. At length, weary of waiting in vain,

I pressed the brass hooks, which were driven into the spinal

marrow, against the iron lattice, in order to see whether

contractions could be excited by varying the incidental circum-

* According

to

a

story which has

often been repeated, but which rests

on

no

sufficient evidence, the frog was one of a number which had been procured for th&

Signora Galvani, who, being in poor health, had been recommended to take a soup,

made of these animals as a restorative.

f Loc. cit., p. 377.

Galvanism, from Galvani to Ohm.

69

stances of the experiment. I observed contractions tolerably often, but they did not seem to bear any relation to the changes in the electrical state of the atmosphere.
" However, at this time, when as yet I had not tried the experiment except in the open air, I came very near to adopt-
ing a theory that the contractions are due to atmospheric electricity, which, having slowly entered the animal and accu-
mulated in it, is suddenly discharged when the hook comes in contact with the iron lattice. For it is easy in experimenting to deceive ourselves, and to imagine we see the things we wish
to see.
" But I took the animal into a closed room, and placed it on an iron- plate ; and when I pressed the hook which was fixed in the spinal marrow against the plate, behold ! the same spasmodic contractions as before. I tried other metals at different hours on various days, in several places, and always with the same result, except that the contractions were more violent with some metals than with others. After this I tried
various bodies which are not conductors of electricity, such as glass, gums, resins, stones, and dry wood ; but nothing happened.
This was somewhat surprising, and led me to suspect that
electricity is inherent in the animal itself. This suspicion was strengthened by the observation that a kind of circuit of subtle nervous fluid (resembling the electric circuit which is manifested in the Leyclen jar experiment) is completed from the nerves to the muscles when the contractions are produced.
" For, while I with one hand held the prepared frog by the hook fixed in its spinal marrow, so that it stood with its feet on a silver box, and with the other hand touched the lid of the box, or its sides, with any metallic body, I was surprised to see the frog become strongly convulsed every time that I
applied this artifice."* Galvani thus ascertained that the limbs of the frog are con-
vulsed whenever a connexion is made between the nerves and
muscles by a metallic arc, generally formed of more than one
*This observation was made in 1786.

70

Galvanism > from Galvani to Ohm.

kind of metal ; and he advanced the hypothesis that the convulsions are caused by the transport of a peculiar fluid from the ' nerves to the muscles, the arc acting as a conductor. To this fluid the names Galvanism and .Animal Electricity were soon generally applied. Galvani himself considered it to be the same as the ordinary electric fluid, and, indeed, regarded the entire phenomenon as similar to the discharge of a Leyden jar. *' The publication of Gralvani's views soon engaged the attention of the learned world, and gave rise to an animated controversy
between those who supported Galvani's own view, those who
believed galvanism to be a fluid distinct from ordinary electricity,
and a third school who altogether refused to attribute the effects to a supposed fluid contained in the nervous system. The leader
of the last-named party was Alessandro Volta (b. 1745, d. 1827), Professor of Natural Philosophy in the University of Pavia, who in 1792 put forward the view* that the stimulus in Galvani's experiment is derived essentially from the connexion of two different metals by a moist body. "The metals used in the * experiments, being applied to the moist bodies of animals, can by themselves, and of their proper virtue, excite and dislodge the electric fluid from its state of rest ; so that the organs of the
* animal act only passively." At first he inclined to combine this
theory of metallic stimulus with a certain degree of belief in such a fluid as Galvani had supposed; but after the end of 17!. '3 he denied the existence of animal electricity altogether.
From this standpoint Volta continued his experiments and worked out his theory. The following quotation from a lettert
which he wrote later to Gren, the editor of the Neucs Journal //. Physik, sets forth his view in a more developed form :
"The contact of different conductors, particularly the metallic, including pyrites and other minerals, as well as charcoal, which I call dry conductors, or of the first class, with moist conductors,
or conductors of the second class, agitates or disturbs the electric
fluid, or gives it a certain impulse. Do not ask in what manner :
f it is enough that it is a principle, and a general principle. This

*Phil. Trans., 1793, pp. 10, 27.

tPhil. Mag. iv (1799), pp. 59, 163, 306.

Galvanism ,

from

Galvani to

Okm.

71

impulse, whether produced by attraction or any other force, is

different or unlike, both in regard to the different metals and to

the different moist conductors ; so that the direction, or at least

the power, with which the electric fluid is impelled or excited, is
A different when the conductor is applied to the conductor B, or

to another C. In a perfect circle of conductors, where either

one of the second class is placed between two different from each

other of the first class, or, contrariwise, one of the first class is

placed between two of the second class different from each other,

an electric stream is occasioned by the predominating force either

to the right or to the left a circulation of this fluid, which ceases
only when the circle is broken, and which is renewed when the

circle is again rendered complete."

Another philosopher who, like Volta, denied the existence of
a fluid peculiar to animals, but who took a somewhat different

view of the origin of the phenomenon, was Giovanni Fabroni, of

Florence (b. 1752, d. 1822), who,* having placed two plates of

different metals in water, observed that one of them was partially

oxidized when they were put in

contact ;

from which he rightly

concluded that some chemical action is inseparably connected

with galvanic effects.

The feeble intensity of the phenomena of galvanism, which

compared poorly with the striking displays obtained in electro-

statics, was responsible for some falling off of interest in them

towards the end of the eighteenth century ; and the last years

of their illustrious discoverer were clouded by misfortune. Being

attached to the old order which was overthrown by the armies

of the French Kevolution, he refused in 1798 to take the oath of

allegiance to the newly constituted Cisalpine Eepublic, and was

A deposed from his professorial chair.

profound melancholy,

which had been induced by domestic bereavement, was aggra-

vated by poverty and disgrace ; and, unable to survive the loss

of all he held dear, he died broken-hearted before the end of

the year.f

* Phil. Journal, 4to, iii. 308 ;

iv.

120 ;

Journal de Physique, vi.

348.

t A decree of reinstatement had been granted, but had not come into operation

at the time of Galvani's death.

72

Galvanism, Jrom Galvani to O/it/i.

Scarcely more than a year after the death of Galvani, the
'
new science suddenly regained the eager attention of philo-

sophers. This renewal of interest was due to the discovery by

Volta, in the early spring of 1800, of a means of greatly increasing

the intensity of the effects. Hitherto all attempts to magnify

the action by enlarging or multiplying the apparatus had ended

in failure. If a long chain of different metals was used instead

of only two, the convulsions of the frog were no more violent.

But Volta now showed* that if any number of couples, each

consisting of a zinc disk and a copper disk in contact, were taken,

and if each couple was separated from the next by a disk of moist-

ened pasteboard (so that the order was copper, zinc, pasteboard,

copper, zinc, pasteboard, &c.), the effect of the pile thus formed

was much greater than that of any galvanic apparatus previously
< introduced. When the highest and lowest disks were simul-

taneously touched by the fingers, a distinct

shock was felt ;

and

this could be repeated again and again, the pile apparently

possessing within itself an indefinite power of recuperation. It

thus resembled a Leyden jar endowed with a power of automati-

cally re-establishing its state of tension after each explosion; with, in fact, " an inexhaustible charge, a perpetual action or impulsion on the electric fluid."
Volta unhesitatingly pronounced the phenomena of the pile to be in their nature electrical. The circumstances of Galvani's

original discovery had prepared the minds of philosophers for this belief, which was powerfully supported by the similarity of

the physiological effects of the pile to those of the Leyden jar, and by the observation that the galvanic influence was conducted only by those bodies e.g. the metals which were already known to be good conductors of static electricity. But Volta now supplied a still more convincing proof. Taking a disk of copper and one of zinc, 'he held each by an insulating handle and applied them to each other for an instant. After the disks had been separated, they were brought into contact with a deli-

* I'hil. Trans., 1800, p. 403.

Galvanism, from Galvani to Ohm.

73

oate electroscope, which indicated by the divergence of its straws

that the disks were now electrified the zinc had, in fact, acquired

a positive and the copper a negative electric charge.* Thus the

mere contact of two different metals, such as those employed in /

the pile, was shown to be sufficient for the production of effects '

undoubtedly electrical in character.

On the basis of this result Volta in the same year (1800)

put forward a definite theory of the action of the pile. Suppose

first that a disk of zinc is laid on a disk of copper, which in turn

rests on an insulating support. The experiment just described

shows that the electric fluid will be driven from the copper to

We the zinc.

may then, according to Volta, represent the state

or " tension " of the copper by the number - J, and that of the

zinc by the number + J, the difference being arbitrarily taken as

unity, and the sum being (on account of the insulation) zero. It will be seen that Volta's idea of " tension " was a mingling of

two ideas, which in modern electric theory are clearly distin-

guished from each other namely, electric charge and electric

potential.
Now let a disk of moistened pasteboard be laid on the zinc,

and a disk of copper on this again. Since the uppermost

copper is not in contact with the zinc, the contact-action does

not

take

place

between

them ;

but since the moist pasteboard is

a conductor, the copper will receive a charge from the zinc.

Thus

the states

will

now

be

represented

by

-
f

for

the

lower

copper, + J for the zinc, and + \ for the upper copper, giving a

zero sum as before.

If, now, another zinc disk is placed on the top, the states will be represented by - 1 for the lower copper, for the lower zinc and upper copper, and + 1 for the upper zinc.
In this way it is evident that the difference between the numbers indicating the tensions of the uppermost and lowest

* Abraham Bennet (b. 1750, d. 1799) had previously shown (Xew Experiments in Electricity, 1789, pp. 86-102) that many bodies, when separated after contact, f are oppositely electrified ; he conceived that different bodies have different attrac-
tions or capacities for electricity.

74

Galvanism , from Galvani to O/im.

disks in the pile will always be equal to the number of pairs of

metallic disks contained in it. If the pile is insulated, the
sum of the numbers indicating the states of all the disks must

be zero; but if the lowest disk is connected to earth, the

tension of this disk will be zero, and the numbers indicating the states of all the other disks will be increased by the same

amount, their mutual differences remaining unchanged. The pile as a whole is thus similar to a Leyden jar ;
when the experimenter touches the uppermost and lowest

disks, he receives the shock of its discharge, the intensity being proportional to the number of disks.
The moist layers played no part in Volta's theory beyond that of conductors.* It was soon found that when the moisture
j.
is acidified, the pile is more efficient; but this was attributed

solely to the superior conducting power of acids. Yolta fully understood and explained the impossibility of
constructing a pile from disks of metal alone, without making use of moist substances. As he showed in 1801, if disks of

various metals are placed in contact in any order, the extreme

metals will be in the same state as if they touched each other

directly without the intervention

of

the others so ;

that the

whole is equivalent merely to a single pair. When the metals

are arranged in the order silver, copper, iron, tin, lead, zinc,

each of them becomes positive with respect to that which

precedes it,

and

negative with

respect

to

that which

follows

it ;

but the moving force from the silver to the zinc is equal to the

sum of the moving forces of the metals comprehended between

them in the series.

When a connexion was maintained for some time between

the extreme disks of a pile by the human body, sensations
were experienced which seemed to indicate a continuous activity

in the entire system. Yolta inferred that the electric current persists during the whole time that communication by con-

* Volta had inclined, in his earlier experiments on galvanism, to locate the seat of power at the interfaces of the metals with the rnoist conductors. Cf. his letter to Gren, Phil. Mag. iv (1799), p. 62.

Galvanism, from Gaivani to Ohm.

75

ductors exists all round the circuit, and that the current is

suspended only when this communication is interrupted.
" This endless circulation or perpetual motion of the electric

fluid," he says, "may seem paradoxical, and may prove inexplicable ; but it is none the less real, and we can, so to

speak, touch and handle it."

Yolta announced his discovery in a letter to Sir Joseph
Banks, dated from Como, March 20th, 1800. Sir Joseph, who

was then President of the Eoyal Society, communicated the

news to William Nicholson (b. 1753, d. .1815), founder of the Journal which is generally known by his name, and his

friend Anthony Carlisle (b. 1768, d. 1840), afterwards a
distinguished surgeon. On the 30th of the following month, Nicholson and Carlisle set up the first pile made in England. In repeating Volta's experiments, having made the contact more

secure at the upper plate of the pile by placing a drop of water

there, they noticed* a disengagement of gas round the con-

ducting wire at this point ; whereupon they followed up the

matter by introducing a tube of water, into which the wires

from the terminals of the pile were plunged. Bubbles of an

inflammable gas were liberated at one wire, while the other

wire became oxidised ;

when platinum wires were used,

oxygen

and hydrogen were evolved in a free state, one at each wire.

This effect, which was nothing less than the electric decomposition of water into its constituent gases, was obtained on

May 2nd, 1800.f

Although it had long been known that frictional electricity

is capable of inducing chemical action,* the discovery of Nicholson and Carlisle was of the first magnitude. It was at

once extended by William Cruickshank, of Woolwich (b. 1745,

i's Journal (4to), iv, 179 (1800) ; Phil. Mag. vii, 337 (1800).
t It was obtained independently four months later l>y J. "W. Hitter. J Beccaria (Lettere deW elettricismo, Bologna, 1758, p. 282) had reduced mercury and other metals from their oxides by discharges ot fractional electricity ; and Priestley had obtained an inflammable gas from certain organic liquids in the same way. Cavendish in 1781 had established the constitution of water by electrically exploding hydrogen and oxygen.

76

Galvanism> from Galvani to Ohm.

d. 1800), who* showed that solutions of metallic salts are also

decomposed by the current; and William Hyde Wollaston

(ft. 1766, d. 1828) seized on it as a testf of the identity of the

electric currents of Volta with those obtained by the discharge
of frictional electricity. He found that water could be decom-

vy posed by currents of either type, and inferred that all differences

between them could be explained by supposing that voltaic electricity as commonly obtained is " less intense, but produced

in much, larger quantity." Later in the same year (1801),

Martin

van

Mar um (ft.

1750,

d.

1837) and

Christian

Heinrich

Pfaff (ft.

1773,

d.

1852)

arrived

at

the

same

conclusion by

carrying out on a large scale} Volta's plan of using the pile to

V charge batteries of Leyden jars.

The discovery of Nicholson and Carlisle made a great impression on the mind of Humphry Davy (ft. 1778, d. 1829), a young Cornishman who about this time was appointed Professor

of Chemistry at the E-oyal Institution in London. Davy at once

began to experiment vvitli Voltaic piles, and in November, 1800,

showed that they give no current when the water between the y pairs of plates is pure, and that their power of action is " in

great measure proportional to the power of the conducting

fluid substance between the double plates to oxydate the

zinc." This result, as he immediately perceived, did not

harmonize well with Volta's views on the source of electricity

in the pile, but was, on the other hand, in agreement with

Eabroni's idea
,

that galvanic effects are

always accompanied by

chemical action. After a series of experiments he definitely
1 concluded that " the galvanic pile of Volta acts only when the

conducting substance between the plates is capable of oxydating

the

zinc ;

and

that, in

proportion

as

a greater quantity of

oxygen enters into combination with the zinc in a given time,

so in proportion is the power of the pile to decompose water

and to give the shock greater. It seems therefore reasonable

* Nicholson's Journal (4to), iv (1800), pp. 187,245: Phil. Mag., vii (1800),

p. 337.

t Phil. Mag., 1801, p. 427.

J Phil. Mag., xii (1802), p. 161.

Nicholson's Journal (4to), iv (1800) ; Davy's Works, ii, p. 155.

Galvanism, from Galvani (o Ohm.

77

to conclude, though with our present quantity of facts we are unable to explain the exact mode of operation, that the </ oxydatioii of the zinc in the pile, and the chemical changes connected with it, are somehow the cause of the electrical effects ^
it produces." This principle of oxidation guided Davy in designing many new types of pile, with elements chosen from the whole range of the known metals.
Davy's chemical theory of the pile was supported by
Wollaston* and by Nicholson,f the latter of whom urged that
the existence of piles in which only one metal is used (with more than one kind of fluid) is fatal to any theory which places the
seat of the activity in the contact of dissimilar metals.
Davy afterwards proposed J a theory of the voltaic pile which combines ideas drawn from both the "contact" and " chemical " explanations. Ho supposed that before the circuit
is closed, the copper and zinc disks in each contiguous pair assume opposite electrostatic states, in consequence of inherent "electrical energies" possessed by the metals; and when a > communication is made between the extreme disks by a wire, the opposite electricities annihilate each other, as in the dis-
charge of a Leyden jar. If the liquid (which Davy compared to the glass of a Leyden jar) were incapable of decomposition, the current would cease after this discharge. But the liquid in
the pile is composed of two elements which have inherent attractions for electrified metallic surfaces : hence arises
chemical action, which removes from the disks the outermost
layers of molecules, whose energy is exhausted, and exposes new metallic surfaces. The electrical energies of the copper and zinc are consequently again exerted, and the process of electromotion continues. Thus the contact of metals is the cause
which disturbs the equilibrium, while the chemical changes continually restore the conditions under which the contact energy can be exerted.
In this and other memoirs Davy asserted that chemical

*Phil. Trans., 1801, p. 427.

t Nicholson'* Journal, i (1802), p. 142.

; Phil. Trans., 1807, p. 1.

78

Galvanism, from Galvani to Ohm.

J affinity is essentially of an electrical nature. " Chemical and electrical attractions," he declared,* "are produced by the same cause, acting in one case on particles, in the other on masses, of matter; and the same property, under different modifications, is the cause of all the phenomena exhibited by
different voltaic combinations."

The further elucidation of this matter came chiefly from - researches on electro-chemical decomposition, which we must
now consider.
A phenomenon which had greatly surprised Nicholson and
Carlisle in their early experiments was the appearance of the products of galvanic decomposition at places remote from each other. The first attempt to account for this was made in 1806 by Theodor von Grothussf (b. 1785, d. 1822) and by Davy,} who advanced a theory that the terminals at which water is decomposed have attractive and repellent powers ; that the pole whence resinous electricity issues has the property of attracting hydrogen and the metals, and of repelling oxygen and acid substances, while the positive terminal has the power of attracting oxygen and repelling hydrogen ; and that these forces are sufficiently energetic to destroy or suspend the usual operation of chemical affinity in the water-molecules nearest the terminals. The force due to each terminal was supposed to
diminish with the distance from the terminal. When the

molecule nearest one of the terminals has been decomposed by the attractive and repellent forces of the terminal, one of its

constituents is liberated there, while the other constituent, by

virtue of electrical forces (the oxygen and hydrogen being in

opposite electrical states), attacks the next molecule, which

is then decomposed. The surplus constituent from this attacks

the next molecule, and so on. Thus a chain of decompositions

and recompositions was supposed to be set up among the

molecules intervening between the terminals.

* Phil. Trans., 1826, p. 383.

f Ann. de Cliim., Iviii (1806), p. 54.

t Bukerian lecture for 1806, Phil. Trans., 1807, p. 1. A theory similar to that

of Grothuss and Davy was communicated by Peter Mark Eoget (b. 1779, d. 1869)

in 1807 to the Philosophical Society of Manchester : cf. Roget's Galvanism, 106.

Galvanism^ from ^Galvani to Ohm.

79

The hypothesis of Grothuss and Davy was attacked in 1825 by Aiiguste De La Kive* (6. 1801, d. 1873) of Geneva, on the ground of its failure to explain what happens when different
liquids are placed in series in the circuit. If, for example, a solution of zinc sulphate is placed in one compartment, and water in another, and if the positive pole is placed in the solution of zinc sulphate, and the negative pole in the water,
De La Rive found that oxide of zinc is developed round the
latter; although decomposition and recomposition of zinc sulphate could not take place in the water, which contained none of it. Accordingly, he supposed the constituents of the decomposed liquid to be bodily transported across the liquids, in close union with the moving electricity. In the electrolysis of water, one current of electrified hydrogen was supposed to leave the positive pole, and become decomposed into hydrogen and electricity at the negative pole, the hydrogen being
there liberated as a gas. Another current in the same way
carried electrified oxygen from the negative to the positive pole. In this scheme the chain of successive decompositions imagined by Grothuss does not take place, the only molecules decomposed being those adjacent to the poles.
The appearance of the products of decomposition at the
separate poles could be explained either in Grothuss' fashion by assuming dissociations throughout the mass of liquid, or
in De La Rive's by supposing particular dissociated atoms
to travel considerable distances. Perhaps a preconceived idea of economy in Nature deterred the workers of that time
from accepting the two assumptions together, when either of them separately would meet the case. Yet it is to this apparent
redundancy that later researches have pointed as the truth.
Nature is what she is, and not what we would make her. De La Rive was one of the most thoroughgoing opponents
of Volta's contact theory of the pile ; even in the case when two metals are in contact in air only, without the intervention
* Annales de Cnimie, xxviii, 190.

80

Galvanism, from Galvani to Ohm.

of any liquid, he attributed the electric effect wholly to the

chemical affinity of the air for the metals.

During the long interval between the publication of the rival

hypotheses of Grothuss and De La Bive, little real progress

was made with

the

special problems

of the

cell but ;

mean-

while electric theory was developing in other directions. One

of these, to which our attention will first be turned, was the

electro-chemical theory of the celebrated Swedish chemist,

Jons Jacob Berzelius (b. 1779, d. 1848).

Berzelius founded his theory,* which had been in one or two

of its features anticipated by Davy,f on inferences drawn from
Volta's contact effects. " Two bodies," he remarked, " which

have affinity for each other, and which have been brought into mutual contact, are found upon separation to be in opposite electrical states. That which has the greatest affinity for oxygen usually becomes positively electrified, and the other

negatively."
This seemed to him to indicate that chemical affinity arises from the play of electric forces, which in turn spring from electric charges within the atoms of matter. To be precise, he supposed each atom to possess two poles, which are the seat of opposite electrifications, and whose electrostatic field is
the cause of chemical affinity.
By aid of this conception Berzelius drew a simple and vivid picture of chemical combination. Two atoms, which are about
to unite, dispose themselves so that the positive pole of one touches the negative pole of the other ; the electricities of these
two poles then discharge each other, giving rise to the heat and light which are observed to accompany the act of combination.! The disappearance of these leaves the compound molecule with the two remaining poles ; and it cannot be dissociated into its constituent atoms again until some means is found of restoring to the vanished poles their charges. Such a means is afforded

* Memoirs of the Acad. of Stockholm, 1812 ; Nicholson's Journal of Nat. Phil.,

xxxiv (1813), 142, 153, 240, 319; xxxv, 38, 118, 159.

t Pnil. Trans., 1807.

J This idea was Davy's.