572 The Evolution of Matter 
thorium-X have been given. These bodies behave differently from 
their parents uranium and thorium, and show all the signs of distinct 
chemical individuality. They are strongly radio-active, while, after the 
separation, the parents uranium and thorium are found to have lost 
some 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, how- 
ever, 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 accom- 
panied 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 geometrical pro- 
