RADIO-ACTIVITY 185 



involved, or must we look deeper for their 

 causes ? 



Three essential pieces of evidence should 

 be considered in this connection. The rate at 

 which radio-active power is gained or lost 

 depends only on, and is always proportional to, 

 the total amount of active material at any instant 

 remaining effective ; it does not depend on the 

 concentration of that material. For instance, if 

 the activity of a quantity of thorium-^, or of 

 radium emanation, be examined, it will be found 

 to decrease during each unit of time by the same 

 fraction of the value it had at the beginning of 

 that interval. If, in the first four days, the 

 activity falls to half its initial value, during the 

 second four days it will fall to half that half- 

 value, or to one quarter of the initial value ; 

 during each successive four days the remaining 

 activity is halved, the process being represented 

 by a curve of the type of those in Figs. 32, 2)3- 

 The rate of decay does not depend on the 

 volume which the material occupies. This mode 

 of change in a geometrical progression, depending 

 only on the total amount of effective material 

 present at the instant, is well known in chemical 

 processes. In such processes it always indicates 

 that the reaction is an alteration going on in the 

 individual molecules, which may either be dis- 

 sociating into simpler molecules, or be suffering 

 a rearrangement of their constituent atoms. Each 

 molecule undergoes this change alone, and does 

 not react with other molecules. If, on the other 

 hand, a change is going on, in which combination 

 or rearrangement between two reacting systems 

 is involved, whether the systems consist of atoms 



