1. It was long ago recognized that matter must be composed of particles
which are driven farther apart by heat and are brought closer together
by cold, thus laying the foundation for the theory of the molecular
composition of matter. But not until the time of Dalton, about a hundred
years ago, was it proved that the molecule itself, the unit of physical
change, is capable of definite division into atoms, the units of
chemical change. This conception of the molecules and atoms as the
ultimate units of which matter is composed maintained its place until
the discovery of radioactivity and its associated phenomena, about
1896; since which time we have definitely ascertained that even the
atoms are separable into still smaller units, and that possibly these
units are _all alike_. On this last possibility, it would surely be a
most amazing fact if such multitudinous "properties" of bodies could be
produced merely by variations in the arrangements of these ultimate
units into atoms, or in some other way which produces vast differences
in properties by combinations of units that are nevertheless mere
duplicates of one another.
As hydrogen is the lightest of the elements, it has been a favorite
theory with scientists that the various elements are all composed of
combinations of hydrogen atoms. But since many of the elements have
atomic weights which cannot be made exact multiples of that of hydrogen,
it has been felt that there must be some other smaller unit than the
hydrogen atom; or else that these hydrogen atoms themselves change in
weight when they combine to form other atoms. But mass seems to be the
one unchangeable characteristic of matter; hence it was felt that any
change of weight is almost unthinkable, and so a solution was sought in
the direction of still further dividing the hydrogen atom, the smallest
unit concerned in chemical change, as then understood. But now the facts
and principles brought to light in connection with the studies of
radioactivity have settled it that we actually do have a much smaller
unit than the hydrogen atom, one of only about 1/1760 its mass, in fact;
and that this smallest of the small things of nature is none other than
a particle of negative electricity, now called an _electron_.
That the atoms of all the elements must have a common unit of
composition, that they behave as if composed of ultimate particles that
may be regarded as duplicates of one another, has long been regarded as
an inevitable conclusion from the Periodic Law of Mendeleef. This law
says that the physical as well as the chemical properties of the various
elements depend upon their atomic weights, or as it is stated in the
language of mathematics, the properties of an element are functions of
its atomic weight. This fact of the variation in the properties of
elements in accord with their atomic weights has been even more
strikingly illustrated by the behavior of discharges of electricity
through rarified gases, as well as by the facts of radioactivity. To
quote the words of Sir J.J. Thompson, "The transparency of bodies to
Roentgen rays, to cathode rays, to the rays emitted by radioactive
substances, the quality of the secondary radiation emitted by the
different elements, are all determined by the atomic weight of the
element."[1]
[Footnote 1: Encyclopædia Britannica, Vol. XVII, 891. Cambridge
Edition.]
Just recently we have had opened up before us a still more intimate
inner-circle view of the composition of matter. H.G.J. Moseley, a young
man only twenty-six years of age, at an English university, devised a
method of examining the spectra of the various elements by means of the
X-rays. He found in this way that the principal lines of these various
spectra are connected by a remarkably simple arithmetical relationship;
for when the elements are arranged in the order of their atomic weights,
they show a graded advance from one to another equal to successive
additions of the same electrical unit charge, thus indicating a real
gamut of the elements that we can run up by adding or run down by
subtracting the same unit of electrical charge. It is pitiable to have
to record that next year this scientific genius was killed in the
ill-fated Gallipoli expedition against Turkey.
Thus in many fairly independent ways we are brought around to this same
idea of a common structure underlying all the many seeming diversities
manifested by what we call matter.
The phenomena of radioactivity were discovered accidentally in 1896 by
the French chemist Becquerel. Many investigators immediately began
working along this promising line, and two years later Madam Curie, in
association with others, discovered the new element radium. Soon it was
discovered that radium and several other substances are continually
giving off radiations at an enormous rate, that no change of chemical
combination, no physical change of condition appears to have the
slightest effect in slowing or increasing this discharge of emanations,
while no scientific apparatus yet devised can detect any change in the
substances left behind either in respect to weight or any other
properties as the result of these enormous losses of energy. Accordingly
some people not unnaturally were ready to draw the conclusion that those
most firmly established laws of physics and chemistry, the laws of the
conservation of energy and of matter, were overthrown by this
astonishing behavior of these newly discovered substances. However, only
a few more years of study and investigation were necessary to prove
that this last conclusion was wholly unwarranted; and to-day these laws
of the conservation of energy and of matter are more firmly established
than ever.
The thing that has gone by the board is the old idea of the atoms as the
indivisible and irreducible minima of the material universe. For not
only do all the radioactive substances give off particles of helium gas
positively electrified, but _all bodies, no matter what their
composition_, can by suitable treatment, such as exposing them to
ultra-violet light, or raising them to incandescence, be made to _give
off electrons_ or negatively charged particles, and _these electrons are
always the same no matter from what kind of substance they come_. In a
somewhat similar way, we always get positively electrified particles of
the mass of the hydrogen atom, or about 1,760 times the mass of the
electron, whenever we send an electric charge through a gas at very low
pressure, _no matter what the kind of gas_. Whether or not these
positive units will yet prove susceptible of being split up into smaller
particles comparable to the electrons, is merely a subject for
conjecture. We have no proof that they will. At the present time what we
call matter seems to be composed of these positive units and of the
electrons which are about 1/1760 as great; and in the present state of
our knowledge these facts suffice to explain all the properties of
matter. Thus we can either say that electricity is composed of matter,
or say that matter is composed of electricity; and human language at
best is such a clumsy vehicle of thought that scientifically and
philosophically the one statement is as correct and as reasonable as the
other.
And probably we shall never be able to learn any more than this. We have
arrived at a sort of box-within-a-box theory of the make-up of matter.
By a very elaborate system of unpacking, or by some violent external
force that makes the inside burst open, as it were, we seem to be able
to make pieces fly off from the atoms, these pieces being then projected
into space with enormous force and velocity. There are theories galore
of the structure of the atom; but as Prof. E.P. Lewis has said, most of
these theories are so impossible as to be absurd, or so speculative that
"they suggest no experimental tests for their validity."[2] Just at
present Rutherford's theory of the structure of the atom is quite
popular. This postulates a nucleus composed of a group of positive units
and electrons, with an excess of the positive charges equal to half the
atomic weight, with an equal number of electrons circulating about this
nucleus in rings. Bohr's theory, which is not very different from this,
has perhaps even more friends, and it is supported by the remarkable
discoveries of the lamented Moseley. But we must not take such theories
too seriously. As Kayser has said, any true theory of the make-up of the
atoms must assume an absolutely full and perfect knowledge of all
electrical and optical processes, and is therefore beyond our dreams. Or
as Professor Planck said in his Columbia lectures, we are not entitled
to hope that we shall ever be able to represent truly through any
physical formulæ the internal structure of the atom.
[Footnote 2: _Nature_, April 5, 1917.]