THE ELECTRON THEORY. 291 



a -rays is in each case opposite to the direction of deflection of 

 the /3-rays, and therefore it is known that the a-particles are 

 positively charged and that the 0-particles are negatively 

 charged. The 7-rays are not deflected by a magnetic field or by 

 an electric field. 



The present hypothesis regarding radio-activity is that the 

 atom of any substance is a system of excessively small nega- 

 tively charged particles, called electrons, revolving around a 

 small positive charge or in a small region containing positive 

 charge, the atom of each element being a characteristic group 

 or system. These systems of electrons (atoms) are supposed 

 to be to some extent unstable, and when instability occurs, 

 the system (atom) collapses into a new configuration, and at 

 the same time expels one or more positively or negatively 

 charged electrons or groups of electrons which constitute the 



^radio-active 

 material 



Fig. 211. 



Showing the relative distances traveled by the 7- wave, by the /3- particle and 

 by the a-particle in a given time. 



a-rays and the /3-rays. According to this hypothesis the 7-rays 

 consist of abrupt ether waves which are produced by the sudden 

 collapse of the atomic structure when instability occurs.* A 

 clear representation of the nature of a-, 0-, and 7-rays is shown in 

 Fig. 211. Imagine an atom of the radio-active material to 



* A very instructive discussion of the electron theory is given by Sir Oliver Lodge 

 in a book entitled Electrons, published by Geo. Bell & Sons, London, 1906. The 

 method of measuring the degree of radio-activity of a radio-active substance is 

 explained in the books of Soddy and Rutherford above referred to. 



