1880.] 



Note on Thermal Transpiration. 



301 



what is in the paper to enable a reader to form a fair judgment of the 

 relative merits of the two methods, I venture to request those inte- 

 rested in the subject to withhold their opinion until they have an 

 opportunity of reading ray paper. In the meantime I can only express 

 my opinion that Professor Maxwell is mistaken in supposing that the 

 results which are obtained from his method are more definite than 

 those to be obtained by mine. 



His method only applies to a particular case, and the equation which 

 he has given is identical with that which 1 have given for this parti- 

 cular case. 



The particular case treated by Professor Maxwell is the extreme 

 limit — when the tube is large as compared with the distances between 

 the molecules ; he does not deal at all with the other limit — when the 

 distances between the molecules are large as compared with the tube. 

 Whereas I have given definite values for the coefficients in both 

 limits, as well as indicating the manner in which the coefficients vary 

 between these limits. 



It so happens that the case in which the tube is large as compared 

 with the molecular distances is one in which the results are too small 

 to be experimentally appreciable, and hence Professor Maxwell's 

 method does not explain any of the actual experimental results. 



In order to explain the experimental results obtained with porous 

 plates, Professor Maxwell has reverted to Graham's assumption that 

 fine plates act as apertures in thin plates, while the coarse plates act 

 like a tube, an assumption which my experiments show conclusively to 

 be unnecessary and erroneous, the only sensible action in either case 

 being that of tubes, and hence the phenomena of porous plates is that 

 of transpiration and not effusion. 



I remain 



Yours truly, 



Osborne Reynolds. 



Professor Stokes, F.R.S., 



Secretary to the Royal Society. 



Note by the Communicator. 



In communicating the above letter to the Royal Society, in accord- 

 ance with Professor Reynolds's wishes, I would beg permission to add 

 a few remarks. 



Professor Maxwell did not profess to treat more than the two 

 extreme cases, constituting what Graham called respectively transpi- 

 ration and diffusion. His statistical method applies, indeed, only to 

 the first of these limits ; but he has distinctly considered the second, 

 following a suggestion of Sir William Thomson's. It is true that at 

 the first limit, as Professor Reynolds remarks, the results are too 



Y 2 



