RADIOACTIVITY 31 



account. The fact is that when an a particle goes through a 

 molecule it goes through one or more atoms in that molecule 

 and its concern is with the inside of each atom it goes through : 

 and it has never to do with more than one atom at a time. This 

 gives a peculiar feature to all the radioactive phenomena, since 

 every other of the radiations shows the same effect. The fi, 7 

 and X rays, as well as the a rays, pay no attention to chemical 

 or physical conditions. 



There is one other a particle effect which we may well 

 consider for a moment. It is well known that a particle when 

 it strikes a phosphorescent screen causes a minute flash at the 

 point of impact. Rutherford and Geiger have greatly developed 

 this method of following the movements of the particles and 

 the latter has shown by means of it that there actually is a 

 small amount of scattering of the a particles on their way 

 through matter. The experiment we have considered above 

 does not show it obviously : and that is not surprising, for 

 Geiger shows that when a stream of particles strikes a plate 

 perpendicularly to its surface, only about one in eight thousand 

 returns. When, however, the velocity falls off very much — 

 it drops to almost nothing at the end of the range — scattering is 

 more pronounced ; and the a particles deviate considerably from 

 the rectilinear path. This is probably the reason of the slope 

 pq in fig. 3. Being much knocked about at the ends of their 

 paths, some of the particles will not go quite so far as others 

 and the ionisation due to the particles does not cease abruptly 

 as it would do if all the paths were quite straight and therefore 

 of the same length. 



Let us now pass on to the consideration of some of the ft rays 

 phenomena. In these we find illustrations of the second of the 

 fundamental principles I have stated above in the existence and 

 the separate existence of the electron. For these rays are 

 actually streams of electrons the same as those which Thomson 

 was the first to examine quantitatively. There can be no doubt 

 as to the identity, for we can proceed by almost complete 

 gradations from the slowest to the fastest moving of the kind 

 and find no break. Thomson worked with the electrons of 

 the cathode rays in the vacuum tube. As is well known, he 

 measured by many methods the charge and mass of the electron 

 and firmly established its existence and its uniformity, no matter 

 from what source it is derived. The /3 ray when examined by 



