THE ROMANES LECTURE
1914
The Atomic Theory
BY
SIR J. J. THOMSON, O.M.
CAVENDISH PROFESSOR OF EXPERIMENTAL PHYSICS IN THE UNIVERSITY OF CAMBRIDGE
DELIVERED
IN THE SHELDONIAN THEATRE JUNE 10, 1914
OXFORD AT THE CLARENDON PRESS
1914

OXFORD UNIVERSITY PRESS
LONDON EDINBURGH GLASGOW NEW YORK TORONTO MELBOURNE BOMBAY
HUMPHREY MILFORD, M.A.
PUBLISHER TO THE UNIVERSITY
1703

T5
THE ATOMIC THEORY
THE theory that matter in spite of its apparent continuity is in reality made up of a great number of very
small particles, is as old as the science of Physics itself,
and was enunciated almost as soon as men began to
reason about physical phenomena. It would, however, be misleading to suppose that there is any very close connexion between the modern Atomic Theory and the views of Democritus and Lucretius. The old theory was in intention and effect metaphysical rather than physical, theological rather than scientific. The physics of two thousand years ago was far too scanty and uncertain to afford any support or test for such a theory ; indeed, if I were called upon to prove to you that Democritus was right when he held that matter was discontinuous, and Aristotle wrong when he said it was not so, I should
have to appeal to facts not one of which was known either to Democritus or Aristotle. The great and invalu-
able service which the Greek atomists have rendered to science is that they were the first to attempt on mechanical principles to explain complicated physical phenomena as the result of combinations of simpler ones ; they pointed out the goal which science is still struggling to reach.
For two thousand years the Atomic Theory itself made no progress, because, though in form a physical theory, it had no real connexion with .physical, phenomena, no facts were known by which it could be tested, and it was too vague to suggest for itself effects which could be put to the test of experiment. It was sterile because it was divorced from experience. It affords a striking
293133

4

The Atomic Theory

proof that a theory can only grow by the co-operation of thought and facts, and that all that is valuable in a physical theory is not only tested, but in most cases suggested, by the study of physical phenomena. In the interplay between mind and matter in scientific discovery, the parts played by the two are, I think, widely different from those usually assigned to them in popular estimation. There is a widespread belief that the mind
itself is desperately speculative, that it is only kept
from wild imaginings by the control of its stolid and prosaic partner, the physical facts. The true state of affairs is, I think, that it is the mind which acts as the
brake in this combination, that the impulsive partner
is the facts, and that these spur on the mind to take leaps which it would shudder at when not under the influence of this stimulus. Nature is far more wonderful
and unconventional than anything we can evolve from our inner consciousness. The most far-reaching generalizations which may influence philosophy as well as revolutionize physics, may be suggested, nay, forced on the mind by the discovery of some trivial phenomenon. To take an example, an improvement in the method of
exhausting air from closed vessels enabled experimenters to send an electric discharge through gas more highly
ratified than had previously been possible. When they
did this they observed that the glass of the vessel shone with a peculiar phosphorescent light : the study of this light led to the discovery of cathode rays, cathode rays led on to Rontgen rays, and the study of those rays started ideas which have entirely changed our conceptions
of matter.
As facts play such a large part in stimulating our imagination and suggesting new ideas, every mechanical improvement in our apparatus, every new method which

The Atomic Theory

5

makes it easier to investigate physical phenomena, affects
not merely the technique of the science, but may originate
ideas which will ultimately revolutionize our philosophy
of the universe. I feel sure, for example, that many of the ideas we now possess regarding atoms and their
structure originated in the study of phenomena which would not have been discovered but for Sir James Dewar's invention for producing very high vacua by means of charcoal cooled by liquid air.
It is not to the theorist alone that scientific ideas owe
their origin ; the inventor of a new piece of apparatus, the mechanic whose skill enables him to construct the
exceedingly sensitive instruments which detect effects so small that they would escape a coarser measure, all play
their part in the progress of scientific ideas.
It is often assumed that the mechanical arts minister
to nothing but material wants, that telephones and telegraphs, motor-cars and aeroplanes merely make life
more luxurious or exciting ; they may do this, but the
engineering skill and activity of which they are the symbol have other and more intellectual effects, and, by the aid they afford us in investigating material
phenomena, may profoundly affect the most philo-
sophical and abstract science. To return, however, to the Atomic Theory : it is not
until the seventeenth century that we find any serious use was made of it for the explanation of physical phenomena, and to that great philosopher, Robert Boyle, who was so closely connected with Oxford, belongs the credit of being the first to use the theory in a way at all analogous to the methods of modern physics. Indeed
Boyle's point of view is quite surprisingly modern. Newton gave the theory his powerful support, and taught that cohesion and chemical affinity were the

6

The Atomic Theory

manifestations of forces between the atoms. One feels,
however, that these great men regarded the idea of
atoms as too vague and speculative to be called upon, except as a last resort : and though Voltaire at the end of the eighteenth century could summarize the state of opinion by saying : ' bodies the most hard are looked upon as full of holes like sieves, and in fact this is what they are. Atoms are accepted indivisible and unchangeable,' it was not until 1801, the date of Dalton's Atomic
Theory, that the conception of the atom played any
considerable part in scientific discovery. Dalton's theory
was based on the proportions by weight of the different elements in various chemical compounds ; he showed that these proportions are exactly those which would exist if each element consisted of a great number of particles, all the particles of any one element being exactly alike, but each element having its own particular kind of particle. He determined the relative weights of the atoms of a number of chemical elements, and he supposed that compound bodies were formed by the union of one or more particles of one element with one or more particles of other elements.
This view gave such a clear-cut and tangible representation of chemical combination, that it was very largely, though not universally, adopted, and caused the conception of the atom to be familiar to every chemist.
Dalton traced the atoms of the different elements in
all their migrations from one compound to another by means of their weight; this was a quality they could
neither change nor disguise; until quite recently, however, this was about the only quality of the atom of which
this could be said. Indeed, with many qualities the way
the individuality of the atom is disguised is exceedingly remarkable, and sceptics had perhaps some excuse when

The Atomic Theory

7

they failed to recognize the atom through all its migra-

tions. Thus a meal of bread and water contains exactly

the same kind of atoms as a draught of a solution of

prussic

acid ;

by merely mixing two colourless liquids

we can get another showing the most vivid colour ; iron

is

intensely

magnetic,

so

are

many

of

its

salts ;

there

"* are others however which, as Professor Townsend has

shown, are non-magnetic, while some of those interesting

compounds of iron and carbon monoxide are actually ' diamagnetic. Does the atom then preserve nothing

intact as it goes from one compound to another except
its weight ? We now know that it does, and we can now

- * give convincing proof of the individuality of the atom

throughout migration. The visible light which the atom

emits changes with the compound, yet, as Professor Barkla

has shown, an atom besides this visible light can also

emit that peculiar kind of invisible light called Rontgen

rays, which only differs from ordinary light in the kind
J of way that blue light differs from red. Barkla has

shown that each kind of atom emits a peculiar type of

Rontgen ray, which remains unaltered, whatever kind of
partner the atom may have. Thus we can detect the

presence of iron, say, in any compound, by studying the

Rontgen rays emitted by that compound ; if it contains

iron we shall find the characteristic Rontgen radiation

of iron present, however complex the compound may

be. With such penetrating agents as Rontgen and

cathode rays at our disposal, other properties which the

atom retains unaltered have been brought to light, such,

for example, as the absorption of these rays when they

pass

through

atoms ;

the

absorption

by a given

atom

is

1 quite independent of any other atoms with which it

may happen to be associated, and depends only on the

quality of the atom itself.

8

The Atomic Theory

The properties of the atom may thus be divided into

two classes ;

in

one class we

have

the properties,

such

as

its weight and its Rontgen radiation, which are intrinsic

to the atom, and which it carries with it unchanged into

any compound of which it may be a constituent ; in the

other class we have the properties, such as the chemical

properties of the atom, which depend upon its environment

and upon the physical conditions, such as temperature,

to which it is subjected. From the point of view of the

structure of the atom, the properties of the second class

depend

upon

the

conditions

of

the

surface

of

the

atom ;

close to the surface there are small negatively electrified
particles, which can be detached from the atom by agents at our disposal, and the properties of the atom modified thereby : the properties of the first class depend upon the structure of the innermost parts of the atom where there are also these negatively electrified particles, which
are, however, so firmly held that they are not loosened
by any chemical treatment it is in our power to apply
to the atom.

For some time after Dalton's enunciation of his theory, no very important advances were made in our knowledge of atoms, but in the second half of the nineteenth century the Atomic Theory was greatly advanced by the work

of Clausius, Clerk-Maxwell, Boltzmann, Joule, Kelvin, and Willard-Gibbs on the Kinetic Theory of Gases.
These philosophers showed that many of the properties
of gases can be explained on dynamical principles if the gas is regarded as a collection of a very large number of small particles in rapid motion. Though some important results as to the size of atoms were obtained in this way, the greater part of the work related to the properties of swarms of atoms, and threw but little light on the constitution of the individual atom. In fact, it was

The Atomic Theory

9

not until quite the close of the nineteenth century, when attention was turned to the study of electrified atoms
instead of unelectrified ones, that our acquaintance with
the atom became at all intimate. The advance made
through the electrification of the atom has been most remarkable it is due to the fact that an unelectrified
;
atom is so elusive that unless more than a million
million are present we have no means sufficiently
sensitive to detect them, or, to put it in another way,
unless we had a better test for a man than we have for /
an unelectrified molecule, we should be unable to find out that the earth was inhabited. The electrified atom
or molecule, on the other hand, is much more assertive, so much so that it has been found possible in some / cases to detect the presence of a single electrified atom ; a billion unelectrified atoms may escape our observa-
tion, whereas a dozen or so electrified ones are detected
without difficulty.
One reason why electrified atoms and molecules are so - much easier to study is that we can subject them to
forces far more intense than any we can apply to unelectrified ones we can exert much more control over
;
them, and force them into situations where their habits
may be observed. For example, if a mixture of different
kinds of electrified atoms is moving along in one streamV then when electric and magnetic forces are applied to the stream simultaneously, the different kinds of atoms ard sorted out, and the original stream is divided up int6 a number of smaller streams separated from each other. The particles in any one of the smaller streams are all, of the same kind.
Thus, if the original stream contained a mixture of
hydrogen and oxygen atoms, it would, by the action of the electric and magnetic forces, be split up into two separate
1705

io

The Atomic Theory

streams, one of which consisted exclusively of oxygen,
the other of hydrogen atoms ; we shall call the streams into which the original stream is split up the electric spectrum of the atoms, and we can by means of it analyse a stream of atoms, just as a beam of light is analysed by sending it through a spectroscope and
observing the different rays into which it is divided.
By means of the electric spectrum we can prove in a very direct and striking way some of the fundamental truths of the Atomic Theory. For example, when we
form the electric spectrum of a mixture of gases, such as
the air, we get a limited number of sharply-divided streams, which show no tendency to merge into each
other. This shows that the gas contains only a few \, kinds of particles, and that all the particles of one kind
have exactly the same mass, for if there had been any variation in the masses the streams would have been

fuzzy. This shows that all the atoms of an element are

alike ;

this

had

sometimes

been

questioned,

and

it

had

been suggested that there might be considerable varia-

tions in the masses of the atoms of the same element ;

ordinary chemical analysis could not settle this question,

for it gives nothing more than the average mass of

billions of atoms. The electric spectrum can be applied

to prove the existence of molecules as well as of atoms,

for when we take the electric spectrum of pure hydrogen,

for example, we find that we get two streams, and that

the mass of the particles in one stream is twice that of

those in the other ;

thus the heavier particles consist of

two of the lighter ones, and in hydrogen there must

be some systems with two atoms, others with one. In

N the majority of gases the spectrum consists of two

streams ;

there are however some gases, such as helium

and mercury vapour, where there is only one stream

The Atomic Theory

II

\ instead of two, showing that in these gases we have atoms but no molecules.

But when we analyse in this way a gas through which an electric discharge is passing, we find along with the
atoms and molecules particles of an altogether different type ; these particles are always charged with negative electricity, and their mass is an exceedingly small fraction, 1/1700, of that of the smallest atom known, the atom of hydrogen. They are so small that their volume bears
to that of the atom much the same proportion as that
between a small pellet and this room. These particles are L called electrons or corpuscles, and no matter what the
nature of the gas may be, whether it is hydrogen, helium,
or mercury vapour, the electrons or corpuscles remain unchanged in quality; in fact, there is only one kind of electron, and we can get it out of every kind of matter. The conclusion is irresistible that the electron or corpuscle
is a constituent of every atom, and that we are able, by forces which we have even now at our command, to

detach it from the atom.

Though the electrons were first detected under the

somewhat artificial and sophisticated condition of a

rarified gas traversed by an electric current, yet, as so

often happens in such cases when once they had been detected, they were found to be of quite common occur-

rence, and to occur in many familiar phenomena. They

are found, for example, round a red-hot piece of metal,

the filament of an electric lamp gives out large quan-

tities ;

they come

out of metals, whether hot

or cold, when

these are reflecting ultra-violet light ; they are given out

spontaneously by radio-active substances; and Haber

has described experiments which indicate that they are

given out during some chemical reactions. There are,

however, many chemical reactions which are not accom-

12

The Atomic Theory

panied by any emission of electrons. Whatever the

source of the electrons may be, they are always the

same ;

some

may be

moving

faster

than

the

others, but

that is the only difference. By observing the behaviour
of the electron under electric and magnetic forces, the

values of its mass and electric charge the quantities

which determine its behaviour under specified con-

ditions have been measured ;

indeed, though the

electron has only lately come under our notice, we know

a good deal more about it than we do of many things

which have been discovered centuries ago. One important

result of these measurements is that the electron or

corpuscle is of the same type when it is ejected with enormous velocities from radio-active substances, as when

it oozes out of a hot body ; this is very strong evidence that it cannot be broken up by any forces we can apply, as these would be insignificant in comparison with those called into play when it is ejected from radium. Since

the electron can be got from all the chemical elements,

we may conclude that electrons are a constituent of

We all atoms.

have thus made the first step towards

a knowledge of the structure of the atom and towards

the goal towards which since the time of Prout many

chemists have been striving, the proof that the atoms

of the chemical elements are all built up of simpler atoms primordial atoms, as they have been called.
As we have proved that the atoms contain these

electrons, the next step is to find out how many there

are in any particular kind of atom. This was first done
by the following method. When Rontgen rays fall on
an electron, the rays are scattered just as light is scattered

by the small particles of carbon in the smoke from

a peat fire, or by the molecules of air in the upper regions

of the atmosphere producing the blue of the sky ; this, by

The Atomic Theory

13

the way, has been used to measure the number of air
molecules in the sky. Now when we know the mass and
charge on an electron we can calculate the amount of hard Rontgen rays scattered by a single electron. Then if we
measure the scattering due to the electrons in an atom,
or in a million atoms, we shall be able to deduce the
number of electrons in the atom. Measurements of the
scattering of Rontgen rays were first made by Barkla, and from his results it follows that the number of electrons
in an atom is roughly proportional to the atomic weight, and that the actual number is not very far from half the atomic weight ; thus in the carbon atom there would be six electrons, in the oxygen atom eight, and so on, while in the lightest atom, hydrogen, there is probably only one. This is a most interesting result when we remember that there is room for 1,700 of these corpuscles in an atom of hydrogen, and that one of the spectra of hydrogen
is of exceptional complexity.
Sir Ernest Rutherford by an entirely different method found that the quantity of positive electricity in an
A atom of atomic weight is equal to the quantity of
negative electricity in A/2 electrons. This also proves that the number of electrons in an atom is half the
atomic weight.
The atomic weights of a great many elements are not
divisible by two, so that the number of electrons in the atoms cannot be exactly equal to half the atomic weight. As the average difference between the atomic weights of
successive elements is about 2, one-half the atomic weight of an element is not very far from its place in the list of elements arranged in order of the atomic weights;
this place is called the atomic number of the element. Mr. van Broek has suggested that the number of electrons in an element is equal to its atomic number, and this

14

The Atomic Theory

view is strongly supported by some remarkably interesting experiments made by Mr. Moseley. If we could be sure that we had a complete list of the elements, that

few, if any, had escaped the vigilance of the chemist, and that all the elements were members of one family,

the atomic number would be the quantity with which we should naturally connect the number of electrons in
the atom : for we may regard each element as derived

from the preceding one by the addition of a primordial

atom containing one electron. There may, however,

be more than one family of elements, the successive
members in each family growing by a common unit,

though the members of one family cannot be changed

into those of the other by the addition or subtraction

of this unit. I think there are reasons for believing that

there are two families of elements for if there were ;
only one family we should expect that the atomic weight of the lighter elements would increase by a common difference. This is not so. If, however, we divide the

lighter elements into two families, those with even and

those with odd atomic weights, we find that in each of

these families the atomic weights do, with very few excep-
tions, increase by the common difference 4, and that in fact we get much greater simplicity and order when we arrange them in two series than when we regard them as
successive members of a single series. This is illustrated

by the following table, which contains the elements

whose atomic weight is not greater than 40 :

He 4

Li

7

Be 9

B ii

C 12

N 14

O 16

F 19

. Ne 20

Na 23

Mg 24

Al 27

The Atomic Theory

15

Si 28 S 32
Ar 40

P 31
Cl 35
K 39

The differences in the atomic weights are the same in
the two series, so that each series may be supposed to grow by the addition of the same kind of primordial //
atom, but one series starts from one kind of atom, the
other from another. The question is, should we not expect the number of electrons in the atom of an element to be connected with the number which represents the
order of the element in the series to which it belongs when the elements are divided into two series, rather
than with its order in a series which contains the whole
of the elements without any rearrangement ? As a matter of fact the difference between the numbers given by these views for the electrons in an atom of one of the heavier
elements would be too small to be detected by any experiment at present within our powers. With the lighter elements, however, it ought to be possible to distinguish between these views, and experiments with this object are at present being made in the Cavendish Laboratory.
The number of electrons in an atom is such a funda-
mental quantity that its determination throws a good deal of light on some of the most keenly discussed problems in Physics and Chemistry, such as the transmutation of the elements and the relation between mass and
weight. Let us begin by considering its connexion with
the first of these questions.

/

TRANSMUTATION OF THE ELEMENTS

The constant difference between the number of electrons

in the atom of one element and that in the atom of the

element next in the series is strong evidence in favour

16

The Atomic Theory

of the view that the atoms of the consecutive elements

differ from each other by the addition of a primordial atom, which apparently is the atom of helium. But though the number of electrons in the atom apparently increases with perfect regularity, the mass of the atom,

at any rate hi the case of the heavier elements, does not

do so. Thus the addition of a constant primordial atom

does not

produce

a

constant

increase

in

the

mass ;

there

must, therefore, be a change in mass when the primordial

atoms coalesce to form the atom of a chemical element ;

and from the values of the atomic weights of the elements

we can get an indication of the change in mass which

has occurred. The consideration of this point leads to

some very interesting results. It is entirely in accor-

dance with electrical principles that some change in mass

should occur when these primordial atoms coalesce ; we

know, for example, that when we push two similarly

electrified bodies together against their mutual repulsion,

the mass of the two increases by an amount proportional

to the work done in pushing them together. When we

know the work spent or liberated in any change of con-

dition, we can calculate the consequent increase or

decrease in mass. In chemical combination heat is

liberated, and there is, therefore, a change in mass, but a calculation shows that even in the cases when the

greatest amount of heat is produced, as for example in the burning of coal, the change in mass is too small to be detected by our most sensitive balances, and though some chemists have devoted a lifetime to the investigation, no change in mass has ever been established as the

result of chemical combination. Since the atomic weights
of the elements show that in their formation a measurable

change of mass has taken place, the changes of energy involved in the formation of the elements must be enormous

The Atomic Theory

17

compared with those liberated in any chemical changes with which we are acquainted. Let us take an example : the atomic weight of chlorine is 35-5 ; this is not a whole number, it differs from the nearest by half a unit; it

follows, therefore, that in the formation of 35-5 grammes

of chlorine there must have been a change of mass of at

least half a gramme. This involves the liberation or

absorption of an amount of energy equal to that possessed

by half a gramme moving with the velocity of light, i.e. 2-25 xio20 ergs. This is about the amount of work

required to keep the Mauretania going at full speed for

a week, and must have been stored up or liberated from

We 35 '5 grammes, or about an ounce of chlorine.

see

that changes in the atom large enough to change the

chemical character of the atom, i.e. to split an atom of

one element up into different kinds of atoms, involve

enormous transformations of energy ; in fact the explosion

of the atom in a few pounds of material might be sufficient

We to shatter a continent.

are living in the midst, nay,

are made

up of

quiescent

volcanoes ;

fortunately their

slumbers are very sound.

Can we break up the atoms by physical means ?

The amount of energy required to break up an atom

has a very important bearing on the problem of splitting

up the atom, in other words the transmutation of the
elements by physical means. We know that the atoms

of the radio-active elements break up spontaneously, and

give rise to atoms of another kind. Thus radium emana-
tion splits up into helium and radium A, and radium A
again splits up. No one, however, has yet been able to

influence the rate at which these transformations take

place by any kind of physical treatment. Intense heat or pressure, and what is much more remarkable bombardment by the a rays given out by the radio-active

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C

18

The Atomic Theory

bodies themselves, seem quite without influence on the

disintegration of the radio-active elements. The bom-

bardment by a rays seems to be the most promising

means of producing atomic transformation, for in this

case the energy of the rays comes from these trans-
formations themselves ' 'tis its own pinion that impels the

steel/ They do not, however, appear to produce any appre-

ciable effect, for the life of a radio-active substance in

a dilute solution, where it is only exposed to a few a rays,

seems to be no longer than the life in a strong solution,

where the substance is bombarded by many rays. I have made many experiments to see if I could split up atoms

of one kind into those of another by exposing them to

electric discharges, bombardment by cathode or positive rays, and other agents ; using the very sensitive method

of positive ray analysis to detect the formation of any

disintegration products ; this method can detect less

than a millionth of a cubic centimetre of a gas at atmo-

spheric pressure. By these means I have been able to

disintegrate the atoms to the extent that I could split

off

from

them

some

of

the

electrons

they

contained ;

from the atom of mercury, for example, I have been

able to detach eight electrons, from hydrogen one electron,

the only one it had. I have never, however, been able to

get any evidence that I regard as at all conclusive that the atom of one element could by such means be changed

into an atom of a different

kind ;

in other words, that by

such means we could produce a transmutation of the

elements.

RATIO OF MASS TO WEIGHT
We have seen that the view, so strongly supported by
recent experiments, that the atoms of the elements are aggregations of simpler systems, involves the admission

The Atomic Theory

19

that losses or gains of mass or weight must occur in the formation of the heavier atoms. But we know that the

ratio of mass to weight is the same for all substances,

from hydrogen, the lightest, up to uranium, the heaviest, and even, as Southern's experiments on uranium and
my own on radium have shown, for radio-active substances. Now in the formation of the heavier atoms

alterations

in mass

must

have

occurred ;

in spite of this

the ratio of weight to mass has not been altered. As

'enormous changes in energy are involved in changes of

mass of the size we are considering far greater than any

we can produce by processes we can use in the laboratory

this is about the severest conceivable test to which we

can put the constancy of the ratio of mass to weight ; that it can stand it is a result of fundamental importance

in the theory of gravitation.
We may ask, does this remarkable constancy in the
ratio of mass to weight, which holds in the case of all
known atoms, hold also for the very much smaller particles, the electrons ? Have these minute negatively
electrified bodies any weight at all, or is, as might be

expected on one of the electrical theories of gravitation, their weight abnormally large in comparison with their mass ? It is perhaps beyond our powers to weigh these

particles, but it is not so hopelessly beyond but that,
with the improvements in technique which we may reasonably expect as the result of experience, we may

entertain hopes of being able to do so before very many

years have elapsed.

In the case of the lighter elements, where the changes
in mass accompanying the formation of the atom may reasonably be expected to be small, we may take the
nearest integer to represent what the mass would have been if there had been no change on aggregation. Taking

2O

The Atomic Theory

hydrogen as the unit, the atomic weights of nearly all the elements up to potassium fall just short of whole numbers this indicates that there has been a diminution
;
A of mass in the evolution of these elements. diminution

of mass means a liberation of energy proportional to it, so that the amount of energy liberated in the formation of these lighter elements will be proportional to the defect of this atomic weight from the nearest integer.
Of the lighter elements whose atomic weights have been determined with great accuracy, magnesium and silicon seem to be the only ones where there are indications of an increase of mass, and in this case the increase is so
slight that a very small error in the determination of the atomic weight would account for these apparent

exceptions.

There are indications that some radical change in the way in which the atom is built up from the primordial atom occurs when we get to atomic weights about 40

or thereabouts. Up to this stage the atomic weights are

expressed by very simple numerical relations which fail

for

the

heavier

elements ;

it

is

at

this

stage

too

that

on

Mendeleeffs system it is necessary to change from the

short period of eight elements, which was sufficient to

represent the cycle of properties of the lighter elements,

to the larger one of sixteen elements to represent those of the heavier ones.

One of the most interesting results of the determination of the number of electrons in the atoms is the

simplicity from one point of view of the hydrogen atom, in which there is only one negative electron. Thus, this
atom is made up of an electron and the equivalent positive charge. Looked at from this point of view, the hydrogen atom is a very simple structure, in fact the simplest that could be built up of electrons and positive

The Atomic Theory

21

electricity ; so that if the atoms of all elements are made up of these constituents there is no room for the existence of an atom lighter than hydrogen, such as that
which has sometimes been suspected to exist in the sun's
corona. The properties of hydrogen are well known and

show no very exceptional simplicity ; thus, for example,

one of its spectra the second spectrum is so complicated

that many thousand different lines have been detected,

and apparently there is no simple relation between the

frequencies of the lines to indicate that they are the

members of a single series like the lines in the first spec-

trum. Is it likely, it may be urged, that such a simple

structure as a single electron and one positive charge

could give rise to a complication as great as this ? But

We is the system so very simple after all ?

must dis-

tinguish between arithmetical and physical simplicity.

The electron and the positive charge produce an electric

field all round them, and an electric field is probably
a very complicated piece of mechanism. We may picture
it in this case as consisting of a large number of lines of force, with one end on the electron and the other on

the positive charge, spreading out into the space round
the atom, and we may also suppose that these lines of force may move about even though their ends are at

rest, and thus vibrate independently of the electrons.
We can easily realize that a bundle of lines of force of
this kind could vibrate in a very great number of ways, far more than would be necessary to account for the

most complicated spectrum yet observed. Before we can get very far in explaining the structure
of the atom, we shall, I am convinced, have to deal with

the question of the structure of the electric field.
It is, I think, possible that an atom may be able to

give out vibrations of almost any period if these are

22

The Atomic Theory

excited in the proper way, say by the impact of cathode

rays possessing a suitable amount of energy, and that the

lines which are actually observed in the spectrum of an

element may be determined by the energy which can be
given to the electrons, which are sucked into the atom by the attraction the atom exerts upon them, rather

than by the inability of the atom to vibrate in other
periods. We may compare an atom to an orchestra

with

a

complete

set

of

instruments ;

the

notes

given

out

will depend upon the players as well as upon the instru-

ments, and the absence of certain notes may be due to

the absence of the appropriate players, and not to that

of the appropriate instrument.
On this view almost any vibration could be excited if

the atom were bombarded with cathode rays of suitable

energy, and the vibrations in the visible spectrum are to be
regarded as excited by the impact of cathode rays in much

the same way as Rontgen rays are excited in a discharge

tube, the difference being merely that the cathode particles
which excite the Rontgen rays have much more energy

than those required to excite the rays in the visible

spectrum, that in fact, in the way it is produced, as

well as in its physical nature, visible light is a special

type of Rontgen ray.
We can produce a system which is still simpler than

the ordinary hydrogen atom, for we can extract the

electron from the atom and get merely the positive

charge left : these positively charged hydrogen atoms

exist in large numbers in the positive rays. The hydrogen

atom, minus its electron, is the simplest atom we can

conceive ;

it

is

much

simpler

than

the normal hydrogen

atom, with its electron intact, and essentially different from it. The investigation of its properties is a matter of very great interest. The comparison of the spectrum

The Atomic Theory

23

of a hydrogen atom which has lost its electron with that of one which has not, is a matter of very great
importance ; unfortunately it is extremely difficult to do it
in a way which is free from ambiguity. On the view just given, the spectrum should be quite different ; indeed we should hardly expect the atom when deprived of its
electron to be able to give out any lines in the visible part of the spectrum. I have recently been able to show that when these positively charged atoms impinge
on other atoms, they give rise to Rontgen rays ; it will be interesting to compare the quality of these rays with those given out by the impact of cathode rays moving either with the same velocity or with the same energy.

THE STRUCTURE OF THE ATOM

We have seen that each atom contains a definite
number of electrons, the number ranging from one for the hydrogen atom to over a hundred for the atom of thorium. The problem of deducing by mathematical consideration the way in which a number of electrons would arrange themselves when in stable equilibrium is one of fundamental importance. In our theoretical investigations of the structure of the atom it is well to keep constantly in our minds the question of the validity of applying to the problem of the individual atom principles which have been established by the study
of the properties of collections of vast quantities of
atoms. In the atom we have to deal with the electron

and the corresponding charge of positive electricity;

these are the units of which all electrical charges are

built up. The laws of electric and magnetic action which

we use in our theoretical investigations are based on the

results of experiments, made not with a single unit of

electricity,

but with

collections

of

millions

of

such

units ;

24

The Atomic Theory

they represent in fact the average effect of millions of
individuals. When, however, we come to the atom, we

have to deal with the effects produced by the individual

electron or positive charge, and not with the average

effect produced by countless numbers of such charges.
Now it may be or it may not be that the average effect

is identical with that produced by each individual, and

it may be or it may not be that a knowledge of the

average is sufficient to solve the problem of the individual.
The statistician is content to know that the average

height of male adults is, say, 5 feet 6 inches, and their

waist measurement 3 feet, but it is evident that such

knowledge would be a very unsatisfactory equipment
for one's tailor. Now the laws of electricity and magnetism
as stated in our text-books are statistical laws, and when

we come to apply them to the atom we are somewhat

in the position of a tailor attempting to fit an individual

with nothing but a knowledge of the average dimensions

We of the whole population to go upon.

must, therefore,

proceed in a somewhat tentative fashion, and try if our statistical knowledge, which is all we have at present, will ensure a fit for the atom ; we need not, however, be very much surprised if the fit is not perfect, and we must,
by the means which fortunately are now at our disposal

for the study of the properties of the electron and the

positive charge, endeavour to supplement our statistical

knowledge by the knowledge of the effect produced by

each individual. I think that the most pressing need at

this stage of the Atomic Theory is the exploration by

experiment

of

the

distribution

of

electrons

in

the

atom ;

when we know this distribution we may be able to see

how we must modify the accepted laws of electrical action

to make them applicable to these small charges.
We may, I think, get a useful lesson by considering

The Atomic Theory

25

for a moment from this point of view a theory of the

atom which, though it is not in very close touch with

physical phenomena, has yet the advantage of being so precisely defined that the properties of its atoms can be deduced by purely mathematical principles. The
theory to which I allude is that known as the ' Vortex Atom Theory of Matter ', which supposes that the Universe consists of an ideal substance known to mathe-

maticians as a perfect fluid. Some portions of this are

supposed to be rotating, the rest not : the rotating parts of the fluid on this theory are the atoms. It can be

shown that any portion of this fluid which once possesses

rotatory motion will never lose it, while if it does not

at

any

instant

possess

it, it

can

never

acquire

it ;

the

atoms on this theory possess at any rate some of the

characteristics of real atoms, as they can neither be

created nor destroyed. The atoms of one substance on

this theory are differentiated from those of another, not

merely by the quantity of the rotating liquid, but also by the speed with which it is rotating. The product of

the angular velocity of rotation and the area of the cross

'

'

section of the rotating fluid is called the strength of

the atom ;

it does

not

change, whatever

vicissitudes

the

atom may experience, and, along with the volume of

the rotating fluid, determines the property of the atom.
Now let us consider some of the properties of the individual

atoms in this theory, remembering that if we took

a collection of a large number of them, the properties

of the aggregate would be those of ordinary matter.

The effective mass of one of these atoms would change

when

it

came

into

collision

with

another

atom ;

this is

because the rotating portion of the atom has to drag

along with itself a considerable volume of the liquid

which is not rotating, so that the effective mass of the

1705

D

26

The Atomic Theory

atom is the mass of the rotating portion, plus the mass of the liquid thus dragged along with it, and as some
of this liquid may be detached from or added to the
atom when it comes into collision with another atom,

the effective mass of the atom will be changed by the

collision. For the same reason, the effective mass of the

atom changes with its velocity the greater the velocity

the smaller being the mass ; so much is this the case that

we have the paradoxical result that the momentum of

the atom decreases as its velocity increases, and that the

more slowly the atom moves the greater is the kinetic

energy. Again, if all the atoms were made of vortices

of

the

same

'
strength ',

we

should

find

that

certain

mechanical quantities would all be integral multiples of

a definite unit, i.e. these dynamical quantities, though

not matter, would yet resemble matter in having an

atomic constitution, being built up of separate indivisible units. The quantity known as ' circulation ' would have

this property; it would always be an integral multiple

of a definite unit, and would thus change by abrupt

steps, and not continuously. When a particle describes

'

'

a circular orbit the circulation is proportional to its

moment of momentum, and we see, that in a theory of

this kind the moment of momentum of particles describ-

ing circular orbits would always be an integral multiple

We of a definite unit.

see from this example that when

we have a structure as fine as that associated with atoms,

we may find dynamical quantities such as moment of

momentum, or it may be kinetic energy, assuming the
atomic quality and increasing or decreasing discon-

tinuously by finite jumps. In one form of a theory

which has rendered great service to physical science

I

mean

Planck's

theory

of

the

'

'

quantum

the changes

from radiant to kinetic energy are supposed to occur

The Atomic Theory

27

not continuously, but by definite steps, as would inevitably be the case if the energy were atomic in structure. I have introduced this illustration from the vortex
atom theory of matter, for the purpose of showing that when we have a structure as fine as that of atoms we
may, without any alteration in the laws of dynamics, get discontinuities in various dynamical quantities, which will give them the atomic quality. In some cases it
may be that the most important effect of the fineness
of the atomic structure will be the production of this atomic quality in some dynamical quantity such as the
kinetic energy. If then we postulate the existence of this property for the energy, it may serve as the equivalent
of a detailed consideration of this structure itself. Thus,
for many purposes (for example, in the elucidation of the remarkable results obtained by Professor Nernst and his
pupils on specific heats at low temperatures, or Mr. Bohr's researches on the distribution of lines in various spectra)
Planck's quantum theory serves as the equivalent of a knowledge of the structure of the atom.
If we assume that the recognized laws of electrical action hold for the small charges carried by the electrified parts of the atom the electrons and the corresponding positive charges we can by the aid of mathematical analysis get 'some idea of the way in which a number of electrons will arrange themselves when in stable
equilibrium. We find that in a symmetrical atom only
a limited number of such electrons can be in equilibrium when arranged on a single spherical surface concentric with the atom : the actual number which can be arranged in this way depends on the distribution of positive
electricity in the inside of the atom. When the number
of electrons exceeds this critical number, the electrons break up into two or more groups arranged in a series

28

The Atomic Theory

of concentric shells. This leads us to the view that the
electrons in an atom, if they exceed a certain number, are divided up into groups, into a series of spherical layers, like the coatings of an onion, separated from each other by finite distances, the number of such layers depending upon the number of electrons in the atom, and thus upon its atomic weight.
The electrons in the outside layer will be held in their
places less firmly than those in the inner layers; they are more mobile, and will arrange themselves more
easily under the forces exerted upon them by other atoms. As the forces which one atom exerts on another
depend on the rearrangement of the electrons in the atom, the forces which a neutral atom exerts on other
atoms what we may call the social quantities of the
atom will depend mainly on the outer belt of electrons.
Now these forces are the origin of chemical affinity, and of
such physical phenomena as surface tension, cohesion,
intrinsic pressure, viscosity, ionising power, in fact of
by far the most important properties of the atom ; and the most interesting part of the atom is the outside belt of electrons. As this belt will be pulled about and distorted by the proximity of other atoms, we should expect that the properties depending on this outer layer of the electrons would not be carried unchanged by an atom through all its compounds with other elements ; they will depend upon the kind of atom with which this atom is associated in these compounds ; they will be what the chemists call constitutive, and not intrinsic. On the other hand the electrons in the strata nearer the centre
of the atom will be much more firmly held ; they will require the expenditure of much more work to remove them from the atom, and will be but little affected by
the presence of other atoms, so that such properties as

The Atomic Theory

29

depend upon these inner electrons will be carried un^ changed by the atom into its chemical compounds. The properties of the real atom are in accordance with these
suggestions. By far the larger number of the properties of the atom are of the constitutive type which we have
associated with the outer belt. of electrons. There are,

however, as we have seen, other properties of the atom

which

are

intrinsic

to it ;

these

we

associate

with the

inner layer of electrons.

FIG. i.
The relation between these two types of properties and the atomic weights are very different. The first type, that depending on the outer layer of electrons, waxes and wanes as we proceed along the list of elements
in the order of their atomic weights; this is illustrated
by the curve in Fig. i, which represents the variation
with the atomic weight of the heat of combination of
the element with chlorine. The relation between an intrinsic property of the atom and its atomic weight is

30

The Atomic Theory

a much simpler one, and is of the kind shown by the
curve in Fig. 2, which represents, according to the experiments of Mr. Whiddington, the relation between the energy required by cathode rays to excite the characteristic Rontgen radiation of an atom and its atomic weight; the same curve will, from the results of the experiments of Mr. Moseley and Mr. Darwin, represent

FIG. 2.

the relation between the frequency of the characteristic

radiation and the atomic weight. The constitutive

properties vary in a quasi-periodic and fluctuating way

with the atomic weight, while the intrinsic ones steadily

increase or decrease, as the atomic weight increases.

This is what we should have expected after our con-

sideration of the properties of groups of electrons when

We in stable equilibrium.

have seen that there cannot

The Atomic Theory

31

be more than a certain number of electrons in any one

layer. Consider how the atom will change as we gradually

increase its population of electrons ; the number in the

outer layer will at first increase, but when it has reached

the critical number no more can be added to it ; any new

added to the atom will now begin to form a new outer

layer, the old outer layer becoming an inner one. With

the addition of more electrons the same process will be

repeated ; the new outer layer will absorb electrons until

it becomes too crowded, when a new outer layer will

split off, and the process be repeated.

The theory of the way in which a number of electrons

arrange themselves suggests that the electrons in the

atom are divided up into a series of rings, one outside

the other. This has been confirmed by experiment, for the

disco very by Professor Barkla of the characteristic Rontgen

radiation has already enabled us to detect two of these

rings in the atoms of the heavier elements and one in
those of the lighter. He showed that when submitted to

appropriate treatment, each atom gives out special kinds

of Rontgen rays ; thus a platinum atom gives out one kind

of ray, a silver atom another, with a longer wave length
than the platinum one. Now the properties of the hardest

rays given out by the different elements are connected

in a very simple way with the atomic weight ; thus

Mr. Whiddington showed that the speed of the slowest

cathode particle which could excite these rays is pro-

portional to the atomic weight, and Mr. Moseley has

shown that the frequency of the vibration is proportional

to

the

square

of

the

atomic

number ;

as

this

number

is

roughly proportional to the atomic weight, the one

relation would follow from the other by Planck's law.

This simple connexion with the atomic weight shows

that these rays arise from similar parts of the atom, and

32

The Atomic Theory

the evidence is very strong that they originate in the innermost ring of electrons. Barkla has shown, moreover, that the heavier elements give out a second characteristic
type of radiation very much softer than the first, which again is connected in a simple way with the atomic

weight of the element. This radiation from elements of small atomic weights

is exceedingly soft, so soft, indeed, that it has not yet
been detected from any element with an atomic weight

less than 90. This softer type of radiation probably originates in the second shell of electrons, counting from
the inside of the atom. By the study of these radiations
we thus get, in the case of the heavier elements, evidence of the existence of two groups of electrons. The radiation from the outer of these groups is so much softer than that from the inner, that if the increase in softness were to continue at the same rate, we should not expect, except
perhaps for elements heavier than lead, to obtain radiations from a third ring which could be detected by the methods
hitherto applied to Rontgen rays. The method thus breaks down as we approach the most interesting part
of the atom.

I think, however, that we may hope before long to have at our disposal methods by which we can produce and investigate Rontgen rays of a much softer type
than those hitherto used. Rontgen rays are usually

generated by shooting rapidly moving electrons against

a solid target ; the greater the speed of the electrons the

harder are the rays they produce. The softest charac-

teristic

radiation yet

detected

is

that

from

aluminium ;

this type of radiation is produced by electrons moving at a speed corresponding to about 3,000 volts, and is so easily absorbed that it is difficult to work with in the
open air. By working inside a very good vacuum, and

The Atomic Theory

33

using a special type of photographic plate, I have, however, been able to photograph radiations produced by electrons whose speed corresponded to only 20 volts, and by increasing the speed of the electrons, to get harder and harder radiations, until at last they were as hard as the kind hitherto studied. The softest radiations

obtained in this way could not get through a film of

collodion, though this was no thicker than a soap bubble ;

they are probably identical with those forms of ultra-violet

light which are called, after their discoverer, Schumann

rays ; with these soft rays we may hope to fill up the

interval between visible light and the hardest Rontgen
rays. These soft Rontgen rays are, I am convinced, likely

to prove of great service in investigating the question of

the structure of the atom ;

they promise to enable us to

determine the number of different groups or rings present

in the atom, and to determine the number of electrons

in each ring. Thus, for example, if we can measure the

absorption of an element for the whole gamut of Rontgen

rays, starting from those characteristic of a heavy
element and going down to Schumann rays, then when-

ever the rays pass through a type corresponding to one

given out by the element, there will be a sudden jump in

the absorption ; by counting the number of these jumps

we could get the number of rings of electrons in the atom.

Or if we measured the emission of Rontgen rays caused

by the impact against the element of cathode rays of

different velocities, there would be similar jumps every

time the velocity of the cathode rays reached the value

which could stimulate a Rontgen ray characteristic of the

element.
We could determine the number of electrons in each

ring by an extension of the method used to determine
the total number of electrons in the atom. When

1705

E

34

The Atomic Theory

Rontgen rays harder than the hardest 'characteristic' radiation of an atom are scattered by the atom every

electron does its full share of the work, so that the

scattering measures the total number of electrons in

the atom ;

if now we take Rontgen rays which, while

softer than the hardest characteristic, are harder than

any of the other types of radiation given out by the

atom, they will not be scattered appreciably by the

electrons in the inner ring, but they will be by all the

other electrons ;

thus the scattering of these rays will give

We us the number of electrons not in the inner ring.

already know the total number of electrons in the atom ;

the difference of these numbers will be the number in

the inner ring. Then if we measure the scattering of

Rontgen rays softer than the next hardest characteristic, but harder than any of the others, we can determine the number of electrons outside the two inner rings; this, since we know the total number of electrons and the

number in the first ring, will give us the number in the

second ring. Thus, by measuring the scattering of softer and softer Rontgen rays, we can determine one after another the numbers of electrons in the rings.

The outer ring of all is the one which gives vibrations slow enough to come within the range of the visible spectrum ; we might expect, therefore, if we measured the

scattering of light well up in the ultra-violet, to be able to determine the number of electrons in the outer ring,

which is in many connexions by far the most important of all. The scattering of light is very closely connected with the refractive index, so that if we know the refractive indices for light going well up in the ultra-violet we could also deduce the number of electrons in this ring. Drude
some time ago, and more recently Erfle and Mr. and Mrs. Cuthbertson, have investigated the number of electrons

The Atomic Theory

35

in this ring on the assumption that it was the only one which influenced the refraction of ordinary light ; the

results they arrived at indicate that there is a close con-
nexion between the number of these electrons and the

chemical valency of the atom. In fact, they suggest that
this number may be equal to the electro-positive valency
of the element. It cannot, I think, be maintained that

the experiments of Drude and others on the indices of

refraction do more than suggest this identity. Many of

the results differ considerably from those which would

We follow from it.

need not, however, I think, attach

any very great importance to these discrepancies, as many

assumptions were made in the course of the work for the

sake of simplicity which may turn out not to have been

well

founded ;

it

was assumed,

for example,

that there

is

only one period in the visible and ultra-violet light portion

of the spectrum which enters into the expression for the

refractive index, and this period was chosen not because
it had been observed in the spectrum, but so as to fit
in with the measurements of the refractive index. We

must remember, too, that one or more of these mobile
electrons in the outer ring may leave the atom when it
enters into chemical combination, and that their arrange-
ment is altered by the proximity of other atoms ; as many of the substances used by Drude were compounds, the number of electrons in the ring may not have been the same as when the atom was in the free state.

The strongest evidence in favour of the close connexion between the number of electrons in the outer

ring and the valency of the elements comes from the chemical properties of the elements, and especially the

various types of chemical compounds they can form.

Very many of these are simply explained by supposing
that near the outside of the atom there are mobile

36

The Atomic Theory

electrons equal in number to the electro-positive valency of the element. The electro-positive valency is the valency when the element is acting as the electro-positive constituent of a compound, and, as Abegg pointed out, is in many cases connected with the electro-negative valency by the rule that the sum of the two valencies is equal to
eight. An atom with n mobile electrons in the outer
ring, or more generally one with an outer, ring of electrons so constituted that when n of its electrons are fixed the
others also lose their mobility, would in its relation to other atoms show the properties which the chemists describe by saying that the electro-positive valency of the atom is n.

I have alluded to several ways of investigating the

structure

of the atom ;

they

one and all

involve great

labour, and any one who has used them must often have

felt what a boon it would have been if we had an eye

which would enable us to have a good look at an atom
and have done with it. Now I cannot say that any such

eye has been invented, but Mr. C. T. R. Wilson has made

some approach to it by a beautiful method by which we

can see, not indeed the individual atom itself, but still

the path of such an atom, and in some cases what is

going on in the atom. The method is based on the

principle that when charged atoms or electrons are

produced in air sufficiently supersaturated with water

vapour, the water condenses on them and nowhere else.

Thus each atom or electron is surrounded by a little drop

of water, and the regions where they are produced are

mapped out by threads of little drops of water resembling

seed pearls ; these can be photographed and studied at

Now leisure.

an electrified atom or electron travelling

through a gas when it strikes against the atoms knocks '

The Atomic Theory

37

some of the electrons out of them, and thus leaves behind

it a trail of electrified wrecks. Mr. Wilson deposits drops

of water on these wrecks, and thus the path of the electri-

fied atom or electron is marked out by a trail of drops of
water which can be seen and photographed. We can

map out in this way the path of even one atom.

I think every worker at the Atomic Theory must have

looked at these photographs with feeling akin to those
of Adams and Leverrier when they first saw Neptune.

Confident as one may be in the truth of a theory, there

are few whose faith is so robust that they do not feel

relieved when they see the conclusions to which they

have been led by theory verified by experiment. Seeing

is believing. Let me quote on this point a sentence by

the great man who fills our thoughts to-day. Roger

Bacon

says,

'
Argument

may

conclude

a

question but

it cannot make us feel certain, except the truth be also

found to be so by experience/
To illustrate what this method can do, let me take two

examples. It has been known ever since the discovery

of Rontgen rays that when these rays pass through a gas

they produce electrified

atoms

and

electrons ;

if we take

by Wilson's method a photograph of air when the Rontgen rays are passing through it, we find that the drops of

water are not uniformly distributed over the photograph,

but are strung together in fine lines giving the appearance

of an untidy spider's web. This shows that when the

atoms are struck by the Rontgen rays some of them

give off electrons moving at a high speed ; the paths of

the electrons are indicated by the fine lines along which the

water drops are arranged. Thus the electrons liberated

by Rontgen rays start off at a speed which carries them
a considerable distance through the air. Now let us take

another case when electrified atoms and electrons are

38

The Atomic Theory

produced in a gas, the case when the gas is traversed by

rapidly

moving

electrons

or

positively

charged

atoms ;

the photographs show that in this case the electrons

liberated from the atoms for the most part start so slowly

that they are unable to travel an appreciable distance

from their origin. For if the electrons knocked out of

the atoms by these moving particles had an appreciable

fraction of the energy of the particles they would pro-

duce ions themselves, and a Wilson photograph would

show branches shooting out from the stem formed by

the drops due to the particle itself. Such branches are

not altogether absent, but they are so sparsely scattered

as to show that the great majority of the liberated

electrons are not set free by direct impact between the

electron and the moving particle, a view which is strongly

supported by the very interesting result obtained by

Lenard and Becker that the velocity with which the

electron is shot out from the atom does not depend to

an appreciable extend upon the speed of the particle

which knocks it out. The laws of ionization by these

moving particles are very different from those by Rontgen

rays ; it is not unlikely that the electrons ejected come

from the outer layer of the atom in the first case and from

an inner layer in the second.

The study of the effects of collisions of electrons or

positively charged atoms with other atoms on which

Professor Townsend and his pupils have done such

valuable work raises very interesting and searching

questions as to the dynamics of the collisions between

these minute bodies. Indeed, as soon as we begin to study

the properties of the atom questions such as these arise

which go to the very root of dynamics and compel us to

examine the fundamental conceptions on which that

science is based. It is quite conceivable that the study

The Atomic Theory

39

of the atom may result in a considerable modification
of the methods of regarding dynamical problems. Though what we know about the atom is but a minute

fraction of what there is to know, some very important conclusions about atoms have been established on what

seems strong evidence in the course of the last few years.
We know, for example, that there are such things as

atoms, that the atoms of an element are all of one kind,
that atoms of different elements contain a common

constituent, the corpuscle or electron about which we

know a good

deal ;

we

know, too, the

number

of

electrons

in an atom. We have strong evidence that the electrons

in the atom are divided into groups, and that some

properties of the atom, those which we associate with

the innermost group, are connected in a very simple
manner with the total number of electrons in the atom ;

that there are other properties, notably the chemical

ones, which change in a rhythmical way with the atomic

weight of the element, and which depend upon the

We electrons near the surface of the atom.

have evidence,

too, that the atoms of the different elements are made up

of simpler systems, and that considerable changes in mass have accompanied the aggregation of these systems. Lastly, we know that there are regions in the atom, probably the most interesting of all, about which we know little or nothing, whose investigation will provide
intensely interesting work for many generations of physicists, who will most assuredly have no reason to be ' mournful that no new wonder may betide '. No
fact discovered about the atom can be trivial, nor fail

to accelerate the progress of physical science, for the
greater part of natural philosophy is the outcome of the structure and mechanism of the atom.