54; Mr. T. S. Thomson on the Diffusion of Gases. 



viously unnoticed fact, that for all gases the moving force of 

 the issuing current is the same, I expressly state, in proceed- 

 ing to apply the formula 



s m = s' W* 

 to diffusion, that "instead of issuing into a vacuum, they should 

 be permitted to diffuse into each other," &c. To meet Mr. 

 Graham's objection, however, I refer to his experiments (p. 74<, 

 Elementary Chemistry), where he says, " A slight deviation 

 from the law is observed in gases which differ in a certain 

 physical property from air, namely in the greater facility with 

 which they may be forced through pores or minute apertures 

 by pressure." 



Mr. Graham finds that carbonic acid and hydrogen rush 

 through stucco into an exhausted receiver, attaining the same 

 internal tension in ten minutes and four minutes respectively. 

 This I conceive to be the reason why, when in a diffusion tube, 

 the air enters against hydrogen more rapidly than against 

 carbonic acid ; the hydrogen comes out more quickly than the 

 carbonic acid, and the conditions of the law being fulfilled, the 

 air rushes in more rapidly, ten minutes to four being very 

 nearly in the proportion of 1277 to 623. 



My immediate object in addressing you, however, is to say 

 that I have a new idea on the subject, which with diffidence I 

 submit to the public. I am only induced to do so because it 

 has in some degree the sanction of the late James Ivory, Esq., 

 to whose letters to me on the subject I refer you. 



I give up the idea of the mutual inelasticity of the particles of 

 different gases. As Mr. Ivory says, "though it is intelligible 

 in words, it is incomprehensible to the understanding." I am 

 inclined to attribute diffusion to an attractive force of a pecu- 

 liar character ; in one sense a chemical force, because it exists 

 only between the particles of dissimilar gases ; in one sense a 

 mechanical force, because it obeys a mechanical law, — the law 

 of diffusion as ascertained by Mr. Graham. The atmosphere 

 of heat surrounding a particle of oxygen is as repulsive to the 

 atmosphere of heat surrounding a particle of hydrogen as it is 

 to that surrounding another particle of oxygen ; but beyond 

 this atmosphere is an incipient chemical attraction of oxygen 

 for hydrogen, not intense enough to determine a direct combi- 

 nation and the formation of water, but still sufficient to deter- 

 mine the particles of oxygen to diffuse themselves gradually 

 among the hydrogen, and vice versa. 



A simple diagram will illustrate this better than any verbal 

 description. Consider two particles of oxygen (i^) presented 

 to two particles of hydrogen (0). 



* i, s' representing the volumes of the interchanging portions of diffusing 

 gases, and ?/?, m' their viasses. 



