nd
distinguished from one another. Two properties are relied upon. One is
the nature of the rays emitted and the other is the duration of the
activity. Of course, knowledge of the physical and chemical properties
is also of great importance whenever obtainable.

Nature of the Radiations

The nature of the radiation is a distinguishing characteristic, though
similarity here does not prove identity of substances. Some emit
[alpha] rays only, some emit [beta] rays, some emit two of the
possible rays, as for instance, [beta] and [gamma], and some emit all
three. The rays may also differ in the velocity with which they are
emitted by different radio-active substances. Thus, in the case of one
substance the [alpha] rays may have a slightly greater or less
penetrating power than those emitted by some other substance, and this
may be true also of the other rays.

Life Periods

The duration of the activity is called the life period. This is
absolutely fixed for each body and furnishes the most important mode
of differentiating among them. It measures the relative stability and
is the time which must elapse before their activity is lost and they,
changing into something else, entirely disappear. The measure usually
adopted is the half-value period. Two hypotheses are made use of:

1. That there is a constant production of fresh radio-active matter by
the radio-active body.

2. That the activity of the matter so formed decreases according to an
exponential law with the time from the moment of its formation.

These hypotheses agree with the experimental results. The decrease and
rise of activity, for example, of uranium and uranium _X_, and also of
thorium and thorium _X_, have been measured, plotted, and the
equations worked out.

Manifestly, a state of equilibrium will be reached when the rate of
loss of activity of the matter already produced is balanced by the
activity of the new matter produced. This equilibrium and the
knowledge of the rate of decrease in general will have little value if
this rate, like chemical changes, is subject to the influence of
chemical and physical conditions. The rate of decrease has been found
to be unaltered by any known chemical or physical agency. For
instance, neither the highest temperatures applicable nor the cold of
liquid air have any appreciable effect.

Equilibrium Series

In order to measure the disintegration of a radio-active body in units
of time so that the rate m