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



by different amounts; thus their velocities of translation will 

 become different, and they will separate. We are thus led to 

 take the view of chemical combination put forward by Clausius 

 and Williamson, according to which the molecules of a com- 

 pound gas are supposed not to alwa3>-s consist of the same 

 atoms of the elementary gases, but that these atoms are conti- 

 nually changing partners. In order, however, that the com- 

 pound gas should be something more than a mechanical mix- 

 ture of the elementary gases of which it is composed, it is 

 evidently necessary that the mean time during which an atom 

 is paired with another of a different kind, which we shall call 

 the paired time, should be large compared with the time 

 during which it is alone and free from other atoms, which 

 time we shall call the free time. If we suppose that the gas 

 is subjected to any disturbance, then this will have the effect 

 of breaking up the molecules of the compound gas sooner than 

 would otherwise be the case. It will thus diminish the ratio of 

 the paired to the free time; and if the disturbance be great 

 enough, the value of this ratio will be so much reduced that 

 the substance will no longer exhibit the properties of a che- 

 mical compound, but those of its constituent elements: we 

 should thus have the phenomenon of dissociation or decom- 

 position. 



The pressure of a gas in any direction is, according to 

 the vortex-atom theory of gases, proportional to the mean 

 value of the product of the momentum and velocity in that 

 direction, just as in the ordinary solid-particle theory. 



Let us now suppose that we have a quantity of gas in an 

 electric field. We shall suppose, as the most general assump- 

 tion that we can make, that the electric field consists of a dis- 

 tribution of velocity in the medium whose vortex-motion 

 constitutes the atoms of the gas; the disturbance due to this 

 distribution of velocity will cause the molecules of the gas to 

 break up sooner than they otherwise would do. Thus the 

 ratio of the paired time to the free time will be diminished. 

 Now, when the atoms are paired, the product of the mo- 

 mentum and velocity for the compound molecule is greater 

 than the sum of the products of the same quantities for the 

 constituent atoms when free, but the pressure in any direc- 

 tion is proportional to the mean value of the product of the 

 momentum and velocity in that direction. Thus each atom 

 will contribute more to the pressure when it is paired than 

 when it is free; and thus, if the ratio of the paired to the free 

 time be diminished, the pressure will be diminished. JSFow, 

 according to any conception which can be formed of the dis- 

 tribution in the medium of the velocity due to the electric 

 field, the variation in the velocity will be greater along the 



