336 BOLTWOOD— RADIOACTIVITY. [Apru 22, 



than three to about eight centimeters. They produce phosphores- 

 cence and chemical action in substances on which they impinge and 

 strongly ionize gases through which they pass. 



Through measurements of their detiection in electric and magnetic 

 fields the beta rays emitted by radioactive substances have been shown 

 to consist of negatively electrified particles or electrons with an 

 apparent mass about one eighteen-hundredth that of the hydrogen 

 atom. The velocity with which they are projected is considerably 

 higher than that of the alpha particles and in some cases exceeds nine 

 tenths the velocity of light. They are capable of penetrating through 

 moderate thicknesses of ordinary matter and for considerable dis- 

 tances in air. They cause phosphorescence and chemical action in 

 substances on which they fall and produce ions in gases through 

 which they pass. 



Although the beta particles emitted by the difl:'erent t}pes of 

 radioactive matter are in all respects identical in nature, they exhibit 

 the same peculiarities with respect to the velocities with which they 

 are initially projected that has been observed in the case of the alpha 

 rays. The velocity of the beta rays from any given radio-element 

 is the same within certain limits for every disintegrating atom of that 

 element, but is difi:'erent from the velocity of the rays emitted by 

 other elements. The velocity of the beta rays is therefore charac- 

 teristic for each of the substances which give rise to this t}pe of 

 radiation and has undoubtedly a significant bearing on the constitu- 

 tion of the radio-atoms. It appears probable from some recent 

 experiments performed by Halin and von Be_\er, in which a magnetic 

 spectrum of the beta rays emitted by certain radioactive substances 

 was obtained, that the transformation of some of the radio-atoms is 

 accompanied by the expulsion of a series of beta particles of differ- 

 ent velocities. These results arc very suggestive of the model atom 

 devised by Sir J- J- Thomson in which the atom was assumed to be 

 built up of concentric shells of electrons revolving with different 

 velocities in circular orbits. It would seem quite possible that in 

 such a system a rearrangement of the parts might result in the expul- 

 sion of electrons from several layers simultaneously. 



The third type of radiation associated with radioactive transfor- 

 mations, known as the gamma rays, is similar to the X-rays and is 



I 



