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- 



