CONTEMPORARY ADVANCES IN PHYSICS 311 



derived.) The charge is usually + e (singly-ionized atom), sometimes 

 + 2e (doubly-ionized atom), rarely + 3e or greater; there is no 

 difficulty in telling which. No one goes to the trouble of determining 

 mass or charge-to-mass ratio absolutely, with the full precision of 

 which the method might be capable; what is actually evaluated is 



54 66 67 68 70 



Fig. 4 — Mass-spectrum of zinc (K. T. Bainbridge). 



the ratio of the mass of each unknown to that of some familiar kind 

 of atom, eventually always the atomic mass of the principal isotope 

 of oxygen. There are various schemes and tricks for facilitating the 

 comparisons, of interest chiefly to those who have some intention of 

 imitating the experiments. Of more general interest is the problem of 

 producing the ions. 



The elements which are gaseous at ordinary temperatures, and those 

 which have compounds that are gaseous at ordinary temperatures 

 (such as carbon in CO and CO2), and the metals which have high vapor- 

 pressures such as mercury — these were analyzed early in the game. 



i li I I 



76 74 73 72 70 



Fig. 5 — Mass-spectrum of germanium (K. T. Bainbridge). 



They are introduced into the discharge-tube, alone or mixed with 

 other gases, and the processes of the discharge ionize their atoms 

 (or the molecules of their compounds, which serve the purpose just 

 as well). Certain others, the alkali metals and the alkaline earths in 

 particular, were conquered through the fact that their ionized atoms 

 stream out of their solid salts when these are heated or bombarded by 

 electrons. The easier cases thus disposed of, it became necessary to 

 lay siege by special artifice to most of those which remained. Constant 

 readers of Nature are acquainted with the letters, generally two to 

 four in a year, in which Aston announces the capture of one fortress 

 after another. Sometimes it is the gift of a sample of some rare 

 element which makes possible the new result, but oftener the con- 

 tribution of some unusual compound of a common element which, 



