i86 PHYSICAL SCIENCE 



or molecules, another law holds ; and the rate of 

 change is found to increase when the material is 

 concentrated into a smaller space, so that the two 

 systems are more closely within reach of each 

 other. In the phenomena we are considering, 

 then, the change involves one system only, what- 

 ever that system may be. 



In examining the further question thus raised, 

 we are confronted at once with the remarkable 

 fact that the radio-activity of a series of com- 

 pounds of any radio-active element is simply 

 proportional to the amount of the element which 

 they contain. The activity of the element is not 

 affected by its state of combination, or by very 

 great changes in the physical conditions, such as 

 temperature, which play a large part in determin- 

 ing ordinary physical or chemical equilibrium. 

 As we have seen, this remarkable result applies 

 not only to the emission of the "rays," but also 

 to the formation of the emanations which proceed 

 from some of the radio-active elements ; the differ- 

 ences in emanating power have been traced to 

 differences in the rate at which the emanations 

 can escape from the various compounds under 

 various conditions. The law of decay of activity 

 shows that one reacting system only is involved ; 

 these further phenomena show that the system 

 does not alter with the changing conditions which 

 are found to affect all known molecular processes, 

 or with the state of combination which affects the 

 physical and chemical properties that control the 

 behaviour of the elements in all other respects. 

 Moreover, as we shall see later, it is possible to 

 calculate the energy liberated by a given amount 

 of radio-active change. This energy is several 



