hat 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 radi