156 PHYSICAL SCIENCE 



ing two unit charges. In neon (atomic weight 

 20.2) two Hnes were found very near together, 

 suggesting atomic weights of 20 and 22. 



This last indication was carried further by 

 F. W. Aston, who improved the apparatus, so 

 that it gave a photographic ''mass spectrum," and 

 obtained most interesting and important results. 

 The double nature of ordinary neon was confirmed. 

 It consists of a mixture of two types of atom, 

 identical in chemical properties, but of different 

 atomic weight. Such bodies were named isotopes 

 by Soddy who discovered instances in another 

 way. Aston, extending the work on mass spectra, 

 proved that manyelements as known to the chemist 

 consist of mixed isotopes. Thus chlorine has an 

 atomic weight of 35.46, and, with similar cases, was 

 always a stumbling block in the road of those who 

 sought to reduce the elements to different com- 

 binations of hydrogen atoms as units of atomic 

 structure. Aston showed at once that its mass 

 spectrum gave two lines, corresponding to atomic 

 weights of 35 and ^J. The riddle of chlorine was 

 solved : it consists of two isotopes, each atomic 

 weight being a whole number. Similar results 

 were obtained with many other elements. 



Amoncr the various ao^encies enumerated at the 

 beginning of this chapter for the production of 

 gaseous ions, special interest attaches to the action 

 of incandescent metals and carbon. Elster and 

 Geitel, Richardson, H. A. Wilson, and others have 

 shown that, as a platinum wire is heated gradually, 

 it begins to emit positive ions at a temperature 

 corresponding to a low red heat. The investiga- 

 tion of the influence of a magnetic force shows 



