RADIOACTIVITY 271 



rays, the discovery and study of Rontgen rays, and the 

 study of gaseous ions. They have led to the idea of the 

 existence of particles which carry positive or negative 

 charges, and which may have dimensions comparable to 

 atomic dimensions, or possibly dimensions considerably 

 smaller. 



The theory of ionization, which has been established to 

 explain the characteristics of electric conductivity in gases, 

 has been recognized as likely to furnish an interpretation 

 of the conductivity acquired by a gas submitted to the 

 action of a radioactive body ; this theory has been applied 

 to the study of radiations emitted by radioactive sub- 

 stances, and constitutes from this point of view a very 

 valuable instrument of research. 



Moreover, the rays of radioactive bodies present analogies 

 to cathodic, positive, and Rontgen rays, and can often 

 be studied by analogous methods. 



It may be said that the discovery of radioactivity 

 occurred at a time when the ground was admirably pre- 

 pared. 



Closely allied to physics and chemistry, and borrowing 

 the methods of work of these two sciences, radioactivity 

 brings to them in exchange, elements of renewal. 



To chemistry it gives a new method for the discovery, 

 the separation, and the study of the elements, as well as the 

 knowledge of a certain number of new elements of very 

 curious properties first of all, radium; and finally, the 

 idea of capital importance of the possibility of atomic 

 transformations under conditions subject to the control of 

 experience. 



To physics, and above all, to modern theories of cor- 

 puscles, it brings a world of new phenomena whose study 

 is a source of progress for these theories. One might cite, 

 for example, the emission of particles carrying electric 

 charges and having a considerable rapidity, whose motion 

 does not obey the ordinary laws of mechanics, and to which 

 one may apply, with the purpose of verifying and develop- 



