RADIUM, ATOMIC ENERGY, AND ATOMIC STRUCTURE 551 



same substance, yet differing in composition. Other cases of a 

 similar nature undoubtedly exist, although not yet encountered 

 in actual practice. 



Isotopes of Common Elements. It has recently been 

 shown by Aston that many common elements also are isotopic, 

 or contain chemically identical atoms of different weight. The 

 method employed by Aston was that of positive ray analysis 

 (Fig. 122). The positive rays from a discharge tube are sorted 



FIG. 122 



out into a thin ribbon by means of the two parallel slits Si and S 2 , 

 and are then passed between the oppositely charged plates PI and 

 P 2 . The rays are deflected towards the negative plate P 2 , and 

 are spread out into an electric spectrum. A portion of this 

 spectrum deflected through a given angle is selected by the dia- 

 phragm D and passed between the circular poles of a powerful 

 electromagnet 0, the field of which is such as to bend the rays 

 back again to fall on a photographic plate placed as shown. If 

 all the rays with a single charge have the same mass, they will 

 converge to a focus at F. If, however, the rays are derived from 

 an element which consists of a mixture of isotopes, each isotope 

 is distinguished by a separate band on the photographic plate, 

 and from the relative position of each band the mass of the atom 

 to which it corresponds can be obtained. 



Chlorine (at. wt., 35.46), examined in this way, showed itself 

 to be a mixture of two isotopes with atomic weights exactly 35 

 and exactly 37. Bromine (at. wt., 79.92) gives isotopes with 



