278 MODERN SCIENCE READER 



the probability of the destruction was, at the same instant, 

 the same for all the atoms of the same matter. It is thus 

 that we interpret the exponential laws of the destruction 

 and the divergences from this law. 



It appears inevitable to admit that the destruction of an 

 individual atom at a given moment results from particular 

 circumstances which the state of this atom and the influence 

 of exterior agents may cause to intervene. 



Thus the determining cause of radioactive phenomena 

 remains still unknown. 



In this book the exposition of the phenomena of radio- 

 activity properly so called has been preceded by an expo- 

 sition of the theory of gaseous ions, and by a resume of the 

 most important knowledge concerning cathodic, positive, 

 and Rontgen rays, and of the properties of electrified 

 particles in motion. This knowledge is indispensable to 

 the study of the subject in hand. A later chapter has been 

 devoted to the description of methods of measurement. 



After the detailed description of the discovery and prep- 

 aration of radioactive substances comes the study of radio- 

 active emanations and of induced radioactivity, and of 

 radiations emitted by radioactive bodies. 



The radioactive substances are afterward classified by 

 families, with the study for each of them of the ensemble 

 of properties and of the nature of radioactive transforma- 

 tions. 



