334 BOLTWOOD— RADIOACTIVITY. [April 22, 



and it is these radiations which led to the discovery of the radio- 

 elements and which particularly distinguishes them from all other 

 types of matter. 



The characteristic radiations emitted by radioactive substances 

 are three in number and are known as the alpha, the beta and the 

 gamma rays. 



For our knowledge of the alpha radiation we are indebted chiefly 

 to the work of Rutherford and his associates, which has shown con- 

 clusively that these rays consist of discreet particles of atomic dimen- 

 sions, projected with high velocities and bearing a positive charge. 

 The earlier investigations were conducted with a view to determin- 

 ing the mass of the particles from the deflections suffered by the 

 rays in electric and magnetic fields of known strengths. A value for 

 the ratio of the charge to the mass (c/iii) was obtained in this man- 

 ner and this led to the conclusion that, if the charge on a particle 

 was the same as that carried by the hydrogen ion in electrolysis, the 

 mass of the particles was approximately twice that of the hydrogen 

 atom. Strong evidence was also obtained that the alpha particles 

 emitted by the difi^erent radio-elements are identical in nature irre- 

 spective of the character of the particular radio-element from which 

 they originate. 



A very ingenious experiment was then devised by Rutherford 

 and Geiger, in which a known fraction of the alpha particles emitted 

 by a radioactive source was allowed to enter a small ionization cham- 

 ber containing a gas at low pressure. Under the influence of a strong 

 electric field the ions formed by the entering particles acquired so 

 high a velocity that additional ions were produced by their collision 

 with neutral gas molecules and the electrical efifect was increased to 

 a point where the action of a single particle could be readily detected. 

 It was shown, moreover, that each of the scintillations appearing on 

 a screen of Sidot's blende placed in the path of the rays corresponded 

 to the impact of a single alpha particle. 



In this manner the actual number of alpha particles emitted by a 

 radioactive substance was accurately counted, and it was found that 

 one gram of radium itself emitted 3.4 X 10^'' alpha particles per 

 second. 



By measuring the electric charge imparted to an insulated plate 



