A HISTORY OF THE THEORIES OF AETHER AND ELECTRICITY : FROM THE AGE OF DESCARTES TO THE CLOSE OF THE NINETEENTH CENTURY E.T. (EDMUND TAYLOR) WHITTAKER A History of the Theories of Aether and Electricity : From the Age of Descartes to the Close of the Nineteenth Century *** E. T. (Edmund Taylor) Whittaker ******ebook converter DEMO Watermarks******* ,NET HARDPRESS ISBN: 978129010626 Published by: HardPress Publishing 8345 NW 66TH ST #2561 MIAMI FL 33166-2626 Email: info@hardpress.net Web: http://www.hardpress.net ******ebook converter DEMO Watermarks******* X LI BR.I 11 11 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* • ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* DU BL IN UN I VERSITY PRESS SERIES . .A HISTORY OF THE THEORIE OF AE'fHER AND ELECTRICITY FROM:: T HE AGE OF DE CARTE TO THE CLO E OF THE NT~~TEEXTH CENTeRY. BY H o/i . c. .D. (.Du hl. ) ; F.R. . ; Roycrl A s ronomt:t· of I rdand. LOXG~IAX , GREEX, A.._~D CO. , 39 PATERXO TER R O W , LONDON , NEW YORK, BOMBAY, A..'lD CALCCTTA. HODGE , FIGGIS, , CO., LTn ., D"CBLIN. 1910. ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* DUBLIN; PRINTBD A 'i . 'J HB UNI VB RSITY PRBSS, BY PONSONBY AND GIRRS. . . . .... ......"" :., . - . : ......: .. ,. :•-:. ........ ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* THE author de ires to record hi gratitude to lV[r. , v. W. Ro SE B ALL, Fellow of Trin ity College, Cambridge., and to Profe or "\V. 1\1:cF. ORR, F.R.S., of the R oyal College of c1ence for I reland; these friends have read the proof- beets, and have made many helpful suggestions and criticism . Thank are also due to the BOARD OF TRINITY COLLEGE,. DUBLIN, for he financial assistance which made pos ible the publication of the work. 236360 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* CONTENTS. CHAPTER I. ~ E THEORY OF THE AETHER IN THE SEVE..."'lTEENTH CE~TURY. l'tiatter and aether, The physical writings of Descartes, Early history of magnetism : Petrus Peregrinus, Gilbert, De:-;cartes, Fermat attacks Descartes' theory of light: the principle of least time, Hooke's undulat-n·y theory: the advance of wave-fronts, Newton overthrows Hooke's theory of colours, -Conception of the aether in the writings of Newton, Newton's theories of the periodicity of homogeneous light, and of fits of easy transmission, The velocity of light : Huygens' Traite de la Glua-hrnleieor,e:R• oheismtehre, ories of the propagation of waves, and of crystalline optics, • Newton shows that rays obtained by double refraction have sides his objections to the undulatory theory, Pag"' 1 2 7 10 11 15 17 20 21 22 28 CHAPTER II. ELECTRIC A.."JD 1\lAGNETIO 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, 32 Gray discovers electric conduction : Desaguliers, • • • 37 The electric fluid, 38 Du Fay distinguishes vitreous and resinous electricity, 39 Nollet's effluent and affluent streams, 40 The Leyden phial, 41 The one-fluid theory: ideas of Watson and Franklin, 42 Final overthrow by Aepinus of the doctrine of effluvia, 48 Priestley discovers the law- of electrostatic force, 50 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* Vlll Contents.' · •J Cavendish, • Michell disoovers the law of magnetic force, . The two-fluid theory : Coulomb, Limited mobility of the magnetic fluids, Poisson's mathematical theory of electrostatics, The equivalent surface- and volume-distributions of magnetism: Poisson's theory of magnetic induction, Green's Nottingham memoir, Page- 51 54 56 58 59 64 65 CHAPTER III. . GALVANISM, FROM: GALVANI TO OHM. Sulzer's discovery, 67 G~~nic~~~~~- ~ Rival hypotheses regarding the galvanic fluid, 70 The voltaic pile, 72 Nicholson and Carlisle decompose water voltaically, 75 Davy's chemical theory of the pile, 76 Grothu" ss' chain, . 78 De La.Rive's hypoth~~;ds, 79 Berzelius' scheme of electro-chemistry, 80 Early attempts _to discover .a connexion between electricity and magnetism, 83 Oersted's experiment: his explanation of it, 85 The law of Biot and Savart, . 86 The researches of Ampere on electrodynan1ics, 87 Seebeck's phenom"enon, 90 Davy's researches on conducting power, 94 Ohm's theory : electroscopic force, 95 CHAPTER IV. ----- " THE LUMINIFEROUS MEDIUM' FR0l\l BRADLEY TO FRESNEL. Bradley discovers aberration, -John Bernoulli's model of the aether, Maupertuis and the principle of least action, Views of Euler, Courtivron, ·MelviU, Young defends the undulatory theory, and explains the colours of thin plates, Laplace supplies a corpuscular theory of double refraction, . 99 100 102 104 105 109 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* Contents. Young proposes a dynamical theory of light in crystals, Researches of l\Ialus on polarization, Recognition of biaxal crystals, Fresnel successfully explains diffraction, His theory of the relatiV"e 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 crystal~, Hamilton predicts conical refraction, Fresnel's theory of reflexion, Page 110 111 113 114 115 121 123 125 131 133 .CHAPTER V. ~THE AETHER AS A.,.'- ELASTIC S0L1n. 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 Ill crystals, 141 143 Cauchy's first theory of reflexion, 145 His second theory of reflexion, 147 The theory of reflexion of J\IacCullagh and Neumann, 148 Green discovers the correct conditions at the boundaries, 151 Green's. theory of reflexion : objections to it, 152 :I\IacCullagh introduces a new type of elastic solid, ,v. Thomson's model of a rotationally-elastic body, 154 157 Cauchy's third theory of reflexion: the contractile aether, . 158 Later work of W. Thomson and others on the contractile aether, 159 Green's first and second theories of light in crystals, 161 Influence of Green, 167 Researches of Stokes on the relation of the direction of vibration of light to its plane of polarization, 168 The hypothesis of aeolotropic inertia, 171 Rotation of the plane of polarization of light by active bodies, 173 l\IacCullagh's theory of natural rotatory power, 175 l\IacCullagh's and Cauchy's theory of metallic reflexion, 177 Extension of the elastic-solid theory to metals, 179 Lord Rayleigh's objection, 181 Cauchy's theory of dispersion, 182 Boussinesq's elastic-solid theory, 185 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* X Contents. CHAPTER VI. FARADAY. Discovery of induced currents: lines of magnetic force, Page .189 Self-induction, 193 Identity of frictional and voltaic electricity: Faraday's views on the nature of electricity, 194 Electro-chemistry, 197 -Oontro·versy between the adherents of the chemical and contact hypotheses, 201 The properties of dielectrics, . 206 Theory of dielectric polarization: Faraday, ,v. Thomson, and Mossotti, 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 Plucker on diamagnetism, 218 CHAPTER VII. THE MATHEMATICAL ELECTRICIANS OF THE l\1IDDLE OF THE NINETEENTH CENTURY. F. Neumann's theory of induced currents the electrodynamic potential, W. Weber's theory of electrons, Riemann's law, . ~oposals to modify the law of gravitation, . Weber's theory of paramagnetism and diamagnetis1n: later theories, Joule's law: energetics of the voltaic cell, Researches of Helmholtz on electrostatic and electrodynam.ic energy, W. Thomson distinguishes the circuital and irrotational magnetic vectors, • His theory of magnecrystallic action, His formula for the energy of a magnetic field, _ Extension of this formula to the case of fields produced by currents, Kirchhoff identifies Ohm's electroscopic force with electrostatic potential, The discharge of a Leyden jar: W. Thomson's theory, The velocity of electricity and the propagation of telegraphic signals, Clausius' law of force between electric charges: crucial experiments, Nature of the current, • • • • • The thermo-electric researches of Peltier and W. Thomson, • 222 225 231 232 234 239 242 244 245 247 249 251 253 254 261 263 264 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* C ont"ents. Xl CHAPTER VIII. ·Gauss and Riemann on the propagation of electric actions, . Analogies suggested by \V. Thomson, l\Iaxwell's hydrodynamical analogy, . The vector potential, Linear and rotatory interpretations of n1agnetism, l\Iaxwell's mechanical model of the electromagnetic field, Electric displacement, Similarity of electric vibrations to those of light. Connexion of refractive index and specific inductive capacity, l\Iaxwell's _memoir of 1864, ·The propagation of electric disturbances in crystals and in metals, . Anomalous dispersion, • The 1\laxwell-Sellmeier theory of dispersion, Imperfections of the electromagnetic theory of light, The theory of L. Lorenz, 1Haxwell's theory of stress in the electric field, The pressure of radiation, l\Iaxwell's theory of the magnetic rotation of light, . • Page 268 269 271 273 274 276 279 281 283 284 288 291 292 295 297 300 303 307 CHAPTER IX. l\foDELS OF THE AETHER. Analogies in which a rotatory character is attributed to magnetism, 1\.1:odels in which magnetic force is represented as a linear velocity, Researches of "\V. Thomson, Bjerknes, and Leahy, on pulsating and oscillating bodies, l\IacCullagh's quasi-elastic solid as a model of the electric medium, The Hall effect. 1\Iodels of Riemann and Fitz Gerald, Vortex-atoms, The vortex-sponge theory of the aether: researches of ,v. Thomson, Fitz Gerald, and Hicks, ~ 310 311 316 318 320 324 326 327 CHAPTER X. THE FOLLOWERS OF l\IAXWELL. Helmholtz and H. A. Lorentz supply an electromagnetic theory of reflexion, Crucial experin1ents of Helmholtz and Schiller, 337 338 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* Xll Contents. Convection-currents: Rowland's experiments, The moving cha1:ged sphere: researches of J. J. Thomson, Fitz Gerald, and Heaviside, Conduction of rapidly-alternating currents, . Fitz Gerald devises the magnetic radiator, Poynting's theorem, Poynting and J. J. Thomson develop the theory of moving lines of force, • Mechanical momentum in the electromagnetic . field, New derivation of Maxwell's equations by Hertz, Hertz's assumptions and Weber's theory, E::itperiments 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 magne.to-optic phenomenon, Rowland's theory of magneto-optics, The rotation of the plane of polarization in naturally active bodies, Page 339 340 344 345 347 349 352 353 356 357 365 367 368 369 370· CHAPTER XI. CONDUCTION IN SOLUTIONS AND GASES, FROI\.I 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 N ernst, Earlier investigations of the discharge in rarefied gases, Faraday observes the dark space, Researches of Plucker, 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, Giese's and Schuster's ionic theory of conduction in gases, J ..J. Thomson measures the velocity of cathode rays, 372 373 374 375 376 379 381 383 386 390 391 393 394 395 397 400 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* Conte>,ts. Discovery of X-rays: hypotheses regarding them, Further researches of J. J. Thon1son on cathode rays: the _ratio -rn/e, Vitreous and resinous electricity, Determination of the ionic charge by J. J. Thomson, Becquerel's radiation: discovery of radio-active substances, Xlll P a g·e 401 404 406 407 408 CHAPTER XII. THE THEORY OF AETHER AND ELECTRONS IN THE CLOSING YEARS OF THE NINETEENTH CENTURY. Stokes' theory of aet-hereal motion near moving bodies, Astronomical phenomena in which the velocity of light is involved, Crucial experiments relating to the optics of 1noving bodies, Lorentz' theory of electrons, . . The current of dielectric convection: Rontgen's experiment, The electronic theory of dispersion, . Deduction of Fresnel's formula from the theory of electrons, Experimental verification of Lorentz' hypothesis, Fitz Gerald's explanation of l\Iichelson's experiment, Lorentz' treatise of 1895, Expression of the potentials in terms of the electronic charges, Further experiments on the relative motion of earth and aether, Extension of Lorentz' transformation : Larmor discovers its connexion with Fitz Gerald's hypothesis of contraction, 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 1netals, The electronic theory of metals, Thermionics, 411 413 416 419 426 428 430 431 432 433 436 437 440 444 449 452 454 456 464 INDEX, 470 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* l\IE1\1ORANDU1'f ON NOTATION. VECTORS are denoted by letters in clarendon type, as E . The three components of a vector E a1·e denoted by E,0 E,,, E,.;: and the magnitude of the vector is denoted by E, so that E 2 = E",,2 + E,,2 + E,.2 • The -vector product of two vectors E and H, which is denoted by [E . HJ, is the vector whose components are (E,,Ka - :I!,,"$,,, E,.Hs - E$,., E$,, - E,,Hz)- 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. The scalar product of E and H is E:,,;ll"" + E,,H,, + E..Hz. It 0 is denoted by (E. H). The quantity is denoted by div E. The vector whose components are ( oE,, oy _ oE,, os' oE"" _ oE,. oy ox' oE,, ox is denoted by curl E. _or) If Vdenote a scalar quantity, the vector whose components ru·e ( - av ox' av - oy' oz is denoted by grad V. The symbol V is used to denote the vector operator whose- o oo components are Differentiation with respect to the time is frequently indicated by a dot placed over the symbol of the ,ariable which is differentiated. ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ~ ~ .., J,# -,IJJ.., 0 J _,,J .., .. -- .., ~ .....,.., .., .., .> , .., .. .,# ,, .,#.., .., .., • THEORIES OF AE'fHER AND ELEC'fRICITY. -·•o+o- CHAPTER 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. l\1odern research, building on this foundation, has shO'wn 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 bet,veen them is an incessant conveyance and transformation. of energy. To the vehicle of this activity the name aether 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 ,vhich fills the interstellar void and the condensations of matter that are scattered throughout it? B ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* 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 Rene Descartes. Descartes was born in 1596, the son of Joachim.· D ·escartes, Counsellor to the Parliament of Brit~any. 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 i:n which he lived, "'.,.ere 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 1nake 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.t 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 la,vs of nature is a task beyond the '# Of the works which bear on our present subject, the .Dioptt·ique and the .Meteorea 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. ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ' in the Seventeenth Century. 3 po,vers o'f one man or one generation, he left to posterity the ,vork 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 h_and sufficient observational data on ·which to base the induction; and as such data ,vere not available in the age of Descartes, he ,vas compelled to deduce phenomena from preconceived principles and causes, after the fashion of the older philosophers. To the inherent ,veakness of this method may be traced the -errors that at last brought his scheme to ruin. The contrast bet,veen the systems of Bacon and Descartes is not unlike that bet,veen the Roman 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 iinmense 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 ,vhich have endured to .our own time. Descartes regarded the ,vorld 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 communicated except by actual pressure or impact. By this a.ssumption 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 ultiinate property of matter. • Since the sun. interacts ·with the planets, in sending them B2 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* 4 The Theory o/ the Aether light and heat and influencing their motions, it followed from Descartes· principle that interplanetary space must be a plenun1.,. occupied by 1natter 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. l\1atter, in the Cartesian philosophy, is characterized not Ly 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, corresponding respectively to the luminous matter of the sun, the. transparent matter of interplanetary space, and the dense,. opaque matter of the earth. cc 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 thatare of such a figure as to fill exactly all the holes and s1nall 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 matternamely, 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 co1nposed of these three forms of matter, as of three distinct ele1nents ; 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 co1nets, 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 ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* ......... 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 inunense vortex formed of the first or subtlest kind of matter.t The vehicle of light in interplanetary space is 1natter of the second kind or element, composed of a closely packed assen1blage of globules ·whose size is intermediate between that of the vortex-matter and that of ponderable matter. The globules of ±he second element, and all the matter of the first element, are .constantly straining a-way from the centres around ,vhich they turn, owing to the centrifugal force of the vortices ;+ so that the globules are pressed in contact ,vith each other, and tend to move outwards, although they do not actually so mo,re.§ 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.If In the .Dioptri,q_iw,9iT vision is compared to the perception of the presence of objects ,vhich 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 be due to the different ,vays in ·which the matter moves.•• In the JJ.feteores,tt the various colours are connected ,vith different rotatory velocities of the globules, the particles ,vhich rotate most rapidly giving the sensation of red, the slo,ver ones of yellow, and the slo,vest 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 specula te on the impression which would have been produced liad the spirality of ue::bulre been discovered before the overthro'\\· of the Cartesian theory of vortices. ! Ibid., §§ 55-59. § Ibid., ~ 63. II Ibid., § 64. "IT .Discours p1·emier. •• P,·incipia, Pa1·t iv, § 195. ti" Discoiws Huitieme. ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* 6 The Theory of the Aether on periodic time is a curious foreshadowing of one of the great discoveries of N e--wton. The general explanati0n of light on these principles ,vas a1nplified by a more particular discussion of reflexion and refraction. The la·w of reflexion-that the angles of incidence and refraction are equal-had been kno·wn to the Greeks; but- the la,v 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 ti1ne.• Descartes gave it as his o,vn; but he seen1s to have been under considerable obligations to Willebrord Snell (b. 1591, d. 1626), Professor of Mathematics at Leyden, ,vho had discovered it experin1entally (though not in the form in ·which Descartes gave it) about- 1621. Snell did not publish his result, but co1n1nunicated it in manuscript to several persons, an9- Huygens affirms that this- n1anuscript had been seen by ])escartes. 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 ,vay as the motion of a ball or- a stone impinging on 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 lavvs as- motion."t 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 ·follows+ : - Let a ball thro,vn fro1n A 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 l..: times as long to describe BI as it took to describe AB. But the co1nponent- * .IJiopt,rique, .IJiscours second. t- 1 bid., .IJiscmtt"s premier. ! .Ibid., .IJiscours second. ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* in the Sevenlleuth Century. 7 of velocity parallel to the cloth must be unaffected by the impact; and therefore the projection BE of the refracted ray must be k times as long as the projection BC of the incident F. E I ray. So if i and r denote the angles of incidence and refraction,. ·we ha-ve . , BE BC . . sin r = BI = k . BA = k sin -i,, or the sines of the angles of incidence and refraction are 1n a constant ratio; this is the law of refraction. Desiring to include all kno,vn phenomena 111. his system,. Descartes devoted some attention to a class of effects which ,vere at that time little thought of, but which were destined to play a great part in the subsequent development of Physics. The ancients ,vere acquainted ·wit:;h the curious properties possessed by t,vo minerals, amber (fJAE1erpov) and magnetic iron ore (11 A.l0o,;; Ma-yvijr,~). 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 kno,vn in the time of the Crusades. Indeed, magnetism ·was one of the few sciences ,vhich progressed during the l\:1iddle Ages; for in the thirteenth century Petrus Pe~egrinus,• a native of ~faricourt 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 • IIis Epistola was written in 1269. ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* 8 The Theo1-y ef 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 t ·wo 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 Natura! Philosophy. The observations of Peregrinus were greatly extended not long before·the time of Descartes by William Gilberd or Gilbertt (b. 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 omnes lineae hujusmodi in duo puncta concurrent si<:ut omnes orbes meridiani in duo concurrunt polos m.undi oppositos." 1' The form. in the Colchester records is Gilberd. t Guliehni Gilberti de l\Iagnete, l\Iagneticisque co1~rib11s, et de magno magnete tellure: London, 1600. An English translation by P. F. l\l'ottelay was published in 1893. ~ ******ebook converter DEMO Watermarks******* ******ebook converter DEMO Watermarks******* in the Seventeenth Century. 9 eYery 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 flu.id matter ,vas 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 ,vay for the more accurate theories that came after. In it~ o,vn 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 ,vane ; for even at Cambridge it was studied long after Newton had published his theory of gravitation ;t and in the middle of the eighteenth century Euler and two of the Bernoullis based the explanation of magnetism on the hypothesis of vortices.+ Descartes' theory of light rapidly displaced the conceptions ,vhich had held sway in the ~liddle Ages. The validity of his explanation of refraction was, ho,vever, called m question by his fellow-countryman Pierre de Fermat (b. 1601, d. 1665),~ and a controversy ensued, which was kept up by the Cartesians long after the death of their master. :F'erm.at • Principia, Part iv, § 133 sqq. t Whiston has recorded that, -having 1·eturne