Mr. T. S. Thomson on the Diffitsion of Gases. 51 



idea may prove to be false ; but should it prove to be true, I 

 have the same undoubted right to impose on that imponde- 

 rable a name, which Davy exercised in giving currency to the 

 word chlorine and its derivatives. The cases are parallel. 

 Has not the universal receipt of that name settled the law ? 



I have now quoted two prominent instances with a view of 

 stamping a proper character on this work. I might have in^ 

 creased these observations to a multitude of other cases which 

 are just as bad; but my purpose is answered. 

 University, New York, Yours, &C., 



May 14th, 1844. John W. Draper. 



VII. Observations on the Diffusion of Gases. 

 By T. S. Thomson, Esq. 

 To Richard Phillips, F.R.S., ^c. 

 Dear Sir, 



THE diffusion of gases is a subject which has agitated the 

 minds of chemical philosophers ever since the element- 

 ary facts were noticed by Dr. Priestley. Dr. Dalton, in his 

 New System of Chemical Philosophy, thus expresses himself, 

 part 1. p. 150 : — " When two or more elastic fluids, whose par- 

 ticles do not unite chemically upon mixture, are brought to- 

 gether, one measure of each, they occupy the space of two 

 measures, but become uniformly diffused through each other, 

 and remain so whatever may be their specific gravities. The 

 fact admits of no doubt; but explanations have been given in 

 various ways, and none of them completely satisfactory. 



" Dr. Priestley was one of the earliest to notice the fact; it 

 naturally struck him with surprise, that two elastic fluids, 

 having apparently no affinity for each other, should not arrange 

 themselves according to their specific gravities, as liquids do 

 in like circumstances. Though he found this was not the 

 case after the elastic fluids had been once thoroughly mixed, 

 yet he suggests it as probable, that if two of such fluids could 

 be exposed to each other without agitation, the one specifi- 

 cally heavier would retain its lower situation. He does not 

 so much as hint at such gases being retained in a mixed state 

 by affinity. With regard to his suggestion of two gases being 

 carefully exposed to each other without agitation, I made a 

 series of experiments expressly to determine the question, the 

 results of which are given in the Manchester Memoirs, vol. i. 

 (New Series). From these it seems to be decided that gases 

 always intermingle and gradually diffuse themselves amongst 

 each other, if exposed ever so carefully ; but it requires a con- 

 siderable time to produce a complete intermixture, when the 

 surface of communication is small. This time may vary from 



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