SOME CONTEMPORARY ADVANCES IN PHYSICS— XI 475 



singly-ionized argon atoms by 30 equivalent volts.'- In the same man- 

 ner, Smyth concluded that the energy of electrons just able to produce 

 doubly-charged mercury ions exceeds by about 9 equivalent volts that 

 of electrons just able to produce singly-charged mercury ions. The 

 method, however, has been used chiefly for studying diatomic gases, 

 and therefore will be mentioned in another section. 



Fig. 5 



Ionization of Molecular Gases ^^ 



The experiments of Thomson and Aston upon the ions proceeding 

 from self-sustaining discharges in molecular gases show that these 

 comprise individual atoms and also molecules of various sorts, each 

 deprived of one or occasionally of more than one electron. Not all of 

 these, however, are produced by the direct and simple agency of a single 

 electron-impact against a normal molecule; some of them result from 

 encounters of ions originally produced in the discharge with molecules 

 which they meet in the gas, either in that region where the discharge is 

 being maintained or in the channel through which they pass to reach 

 the analyzing fields. This stands out very clearly in such experiments 

 as one performed by A. J. Dempster, who projected 800-volt electrons 

 into hydrogen gas and determined the relative abundance of the ions 

 H+, H2+ and H3+ arriving at his collecting-electrode after passing 

 through a certain distance in the gas. At a gas-pressure amounting to 

 .01 mm. Hg, the H3+ ion was the most plentiful of all and the other two 

 not far behind; at .0017 mm. Hg both the H+ and H3+ ions were 

 definitely less abundant than H2+, and below .0005 mm. the H2+ ion 



12 Actually he obtained 17.3 volts for the one critical potential, 47.4 for the other, 

 and assumed that the difference between 17.3 and the accepted value of 15.2 for the 

 first ionizing-potential of argon is due to contact potentials and other influences 

 affecting each of the observed critical potentials equally. 



" For a general bibliography of this subject see T. R. Hogness & E. G. Lunn, Phys. 

 Rev. (2) 26, pp. 44-55, 786-793 (1925); also V. Kondratjeff, ZS. f. Phys. 22, pp. 1-8 

 (1924) and 31, pp. 535-541 (1925). 



