on, or, as it is expressed, 1/2 Ae.

There is only one lighter atom than helium, namely, hydrogen, which
has a mass only one-fourth as great. When alpha particles are
discharged into hydrogen, a few of the latter atoms are found to be
propelled to a distance four times as great as that reached by the
alpha particles.

Stopping Power of Substances

Parallel with the experiments mentioned, there is what is called
the stopping power of substances. This means the depth or thickness
of a substance necessary to put a stop to the course of the alpha
particles. This gives the range of the alpha particles in such
substances and is connected in a simple way with the atomic weight,
that is, it is again fixed by the mass of the opposing atom. This
stopping power of an atom for an alpha particle is approximately
proportional to the square root of its atomic weight.

Considering gases, for instance, if the range in hydrogen be 1,
then the range in oxygen, the atomic weight of which is 16, is only
(1/16)^{1/2} or 1/4. Generally in the case of metals the weight of
matter per unit area required to stop the alpha particle is found to
vary according to the square root of the atomic weight of the metal
taken.

CHAPTER VI

RADIO-ACTIVITY AND CHEMICAL THEORY

Influence upon Chemical Theory

It can easily be seen that the revelations of radio-activity must have
a far-reaching effect upon chemical theory, throwing light upon, and
so bringing nearer, the solution of some of the problems which have
been long discussed without arriving at any satisfactory solution. The
so-called electro-chemical nature of the elements will certainly be
made much clearer. The changes in valence should become intelligible
and valence itself should be explained. A fuller understanding of the
ionization of electrolytes also becomes possible. As these matters are
debatable and the details are still unsettled, it is scarcely
appropriate to give here the hypotheses in detail or to enter into any
discussion of them. But the promise of solution in accord with the
facts is encouraging.

The Periodic System

Such progress has been made, however, in regard to a better
understanding of the Periodic System that the new facts and their
interpretation may well be given. No reliable clue to the meaning of
this system and the true relationship between the elements had been
found up to the time when new light was thrown upon it by the
discoveries of radio