of their radio-activity. If the
X-substances be kept, their radio-activity decays, while that of the
uranium or thorium from which they were obtained gradually rises to the
initial value it had before the separation. At any moment, the sum of
the radio-activity is constant, the activity lost by the product being
equal to that gained by the parent substance. These phenomena are
explained if we suppose that the X-product is slowly produced in the
substance of the parent, and decays at a constant rate. Uranium,
as usually seen, contains a certain amount of uranium-X, and its
radio-activity consists of two parts--that of the uranium itself, and
that of the X product. When the latter is separated by means of its
chemical reactions, its radio-activity is separated also, and the rates
of decay and recovery may be examined.

Radium and thorium, but not uranium, give rise to radio-active gases
which have been called emanations. Rutherford has shown that their
radio-activity, like that of the X products, suffers decay, while
the walls of the vessel in which the emanation is confined, become
themselves radio-active. If washed with certain acids, however, the
walls lose their activity, which is transferred to the acid, and can be
deposited by evaporation from it on to a solid surface. Here again it
is clear that the emanation gives rise to a radio-active substance which
clings to the walls of the vessel, and is soluble in certain liquids,
but not in others.

We shall return to this point, and trace farther the history of the
radio-active matter. At present we wish to emphasise the fact that, as
in other cases, the radio-activity of the emanation is accompanied
by the appearance of a new kind of substance with distinct chemical
properties.

We are now in a position to consider as a whole the evidence on the
question of the source of radio-active energy.

(1) Radio-activity is accompanied by the appearance of new chemical
substances. The energy liberated is therefore probably due to the
associated chemical change. (2) The activity of a series of compounds is
found to accompany the presence of a radio-active element, the activity
of each compound depends only on the contents of the element, and is
independent of the nature of its combination. Thus radio-activity is a
property of the element, and is not affected by its state of isolation
or chemical combination. (3) The radio-activity of a simple transient
product decays in a geo