432 Mr. J. J. Thomson on a Theory of 



This view of the electric discharge indicates a relation 

 between the electric strength of a gas and its chemical pro- 

 perties; for in order to make the spark pass through an ele- 

 mentary gas we have to decompose the molecules into atoms: 

 thus the stronger the connexion between the atoms in the 

 molecule, the greater the electric strength. Thus, for example, 

 we should expect that the atoms of nitrogen are much more 

 firmly connected together in the molecule than the atoms 

 of hydrogen, as the electric strength of nitrogen is much 

 greater than that of hydrogen. Unfortunately we seem to 

 know very little about the strength of connexion between the 

 atoms in the molecule; it would, however, be interesting to 

 try whether a gas whose molecules, like those of iodine vapour, 

 are easily dissociated into atoms would be electrically weak. 

 In many cases, of course, the decomposition of the gas on the 

 passage of the spark is very evident; a common way of decom- 

 posing a gas being to pass sparks through it. We might, 

 however, have chemical decomposition without being able to 

 detect the products of it ; for these might recombine as soon 

 as the disturbance produced by the electric field was removed. 

 In the case of an elementary gas, the splitting-up of the mo- 

 lecules into atoms will effect the same purpose as the decom- 

 position of the compound gas — i. e. the exhaustion of the 

 electric field. Thus, according to the view we are now dis- 

 cussing, chemical decomposition is not to be considered merely 

 as an accidental attendant on the electrical discharge, but as 

 an essential feature of the discharge, without which it could 

 not occur. 



Let us now consider what effect rarefying the gas would 

 have upon its electrical strength. In a rare gas the mean 

 distance between the molecules is greater than in a dense one; 

 and if the temperature be the same in both cases, and conse- 

 quently the mean velocity of the molecules the same, the ratio 

 of the free to the paired time will be greater for the rare than 

 for the dense gas; for the free atoms will, on an average, be 

 longer in meeting with fresh partners. Thus the rare gas will 

 be nearer the state in which it begins to suffer dissociation 

 than the dense gas, and thus it will not require to be disturbed 

 so violently as the dense gas in order to increase the ratio of 

 the free to the paired time to its dissociation value ; and thus 

 the intensity of the field necessary to produce discharge 

 would be less for the rare gas than for the denser one: 

 in other words, the electric strength would diminish with the 

 density; and this we know is the case. It is now generally 

 admitted that rare gases are more easily dissociated than 

 denser ones. In fact Sir C. W. Siemens takes this as the basis 

 of his theory of the Conservation of Solar Radiation, as Jie 



