122 



NATURE 



[Dec, 13, 1877 



sjQfs : — "Then, I imagine, the flow of heat through the gas 

 will take place as though there were, in contact with each solid 

 surface, a layer of gas whose temperature is throughout the same 

 as thai of the contijiuous solid, and whose thickness is equal (or 

 at least proportional) to the mean length of path of the molecules." 

 Without these layers of uniform temperature or whatever may 

 produce an equivalent effect it follows directly from Pi of. Foster's 

 reasoning that the rate at which heat is communicated is, as I 

 maintain it is, independent of the density, whereas if there were 

 any such layers I should at once admit the force of Prof. Foster's 

 reasoning. The who'e questiiar turns therefore on the existence 

 of ihese layers of uniform temperature. 



Now what evidence of such layers have we ? No experimental 

 evidence certainly ; and not only has the kinetic theory not as 

 yet been appled to expla-n their ex'stence but it is easy to 

 demonstrate that according to this theory no such layers or any 

 equivaleiit can exist. For in order that the condition of heat 

 may remain unaltered it is necessary that the rate at which heat 

 is transmitted across all suraces parallel to the solid surfaces 

 which can be drawn through the gas should be the same. And 

 the rate at which heat is transmitted is for small variations of 

 temperature propnnional to the degradaii'm of tempe'ature, 

 hence if there were a layer of unilorm temperature no heat could 

 be transmitted. 



It is surely incumbent on Prof. Foster in assuming the existence 

 of thtse layers to give some sort of proof in support of his 

 assumption, but not one word does he say ! 



I cannot allow this to pass without pointing out that the 

 description which Mr. Stoney has given of my viev/ is grossly 

 wrong and is certainly not to be gathered from anything I have 

 written. Mr. Stoney carefully turns my position. He makes 

 out that I have explained the action in question as arising from 

 convtction currents, whereas I have from first to last maintained 

 that it is these currents which oppose and eventually overcome 

 the action. He makes out that my theory takes no account of 

 molecular motion, whereas, in truth, it takes no account of any- 

 thing but molecular motion, the effect of the expansion of the 

 gas being so obviously trivial that I have never even men- 

 tioned it. 



Your readers may judge of this by comparing the first of the 

 following quotations, which is from Mr. Stoney's letter, with the 

 others which are from my own papers, and are the only expres- 

 sions, not mathematical, which I have given of my views as to 

 fiction in the question : — 



Mr. Stoney's. 

 "Prof. Osborne Reynolds's 

 explanation is based on the fact 

 that when a disc with vertical 

 sides is heated on one side and 

 exposed to a gas, a convection 

 current sets in, which draws a 

 continuous supply of cold gas 

 into contact with the hot surface 

 of the disc. As this cold gas 

 reaches the disc it is expanded, 

 and thus its centre of gravity is 

 thrown further from the disc. 

 Accordingly, the disc, if freely 

 suspended, will move in the 

 opposite direction so as to keep 

 the centre of gravity of the gas 

 and disc in the same vertical 

 line as before, and, if not freely 

 suspended, will suffer a pressure 

 tending to make it move in that 

 direction. If I have understood 

 Prof. Reynolds aright, this is 

 both a correct and full descrip- 

 tion of his explanation as last 

 piesented." 



My Own. 

 " Whenever heat is commu- 

 nicated from a hot surface to 

 gas, the particles which im- 

 pinge on the surface will re- 

 bound with a greater velocity 

 than that with which they ap- 

 propriate ; and consequently 

 the effect of the blow must be 

 greater than it would have been 

 had the surface been of the 

 same temperature as the gas. 



" And, in the same way, 

 whenever heat is communicated 

 from a gas to a surface, the 

 force on the surface will be 

 less than it otherwise would 

 be, for the particles will re- 

 bound with a less velocity than 

 that at which they approach.' 



" These forces arise from 

 the communication of heat to 

 or from the surface from or to 

 the gas. These forces will be 

 directly proportional to the rate 

 at which the heat is communi- 

 cated ; and since this rate has 

 been shown by Prof. Maxwell 

 to be independent of the den- 

 sity of the gas, these forces will 

 be independent of the density 

 of the surrounding medium, 

 and their effect will increase as 

 the density and convection- 

 currents diminish." ^ 



Proceedings, Royal Society, 1874, p. 407. 

 Pkil. i)/(?^., November, 1874, p. 3. 



The first of the quotations from my papers is followed by a 

 mathematical expression on which I have depended for com- 

 pleteness, and from thi-i expression, in which neither convection 

 currents nor the expansion of the g*s have any place whatsoever, 

 it follows that whenever heat is steadily diffusing into or through 

 a gas, the momentum transmitted across any surface in the 

 direction in which the heat is diffusing will be greater than that 

 transmitted in the opposite direction by a quantity proportional 

 to the rate at which the heat diffuses, divided by the square root 

 of the absolute Temperature of the gas. 



As to the value of what follows in Mr. Stoney's letter, I must 

 leave it to your readers to decide. He proceeds to claim that 

 his own theory has the advantage of being based on molecular 

 mot'on«, he says : — 



*' My explanation, on the other hand, is based on molecular 

 motions which go on in the gas without causing any molar 

 mo'ion, and is independent of convection currents." 



Then having thus attributed to me an explanation, I never 

 even thought of offering, and having assumed the true base 

 of my theory as alone belonging to Ins. he proceeds to show 

 wherein I am wrong. And ia every subsequent position which 

 he attributes to me, he is as wrong as he is in h's first statement. 



Under these circumstances it would be useless for me to enter 

 upon questions a>. to how far "diffusion," ace irdmg to the kinetic 

 theory may be moie "sluggish " than Mr. Stoney's " penetration, ' 

 or to discuss lurther the possibility of his " Crookes's layers." 



In my last letter I showed that the condition of a gas which 

 Mr. Stoney called a " Crookes's layer" was impossible, and I do 

 not see that Mr. Stoney has improved his position by showing 

 that he had arrived at the \ ossibility of the condition by making 

 the false assumption '^ that gas is a perfect non-conductor of heat.'" 



Wherein Mr. Stoney's views are at variance with the results 

 of the laborious investigations of Maxwell, Clausius, Tiiomson, 

 and others, he may best convince himself by referring to the 

 works of the.'C authors. Until he has read my papers and 

 explained the discrepancies between his views and the generally- 

 accepted laws of gase-, I do not see that we have any common 

 grrund for discussion. OSBORNE Reynolds 



November 30 



Mr. Crookes and Eva Fay 



If Mr. Wallace had read my letter in Nature of November 

 29 with a little more attention, he would have seen that I did 7iot 

 refer to the Daily Teleg7-aph "as an authority in a matter of 

 scientific inquiry," but that the account I gave of Mr. Crookes's 

 "scientific tests " was given in Air, C.'s own communication to 

 the ' Spiritualist,' which would have been reproduced without 

 abridgment if the columns of Nature could have admitted it. 



What I hold myself pledged to show (in Nature, if it 

 pleases, as well as in the new edition of my Lectures) is that the 

 " tying-down by electricity " described by Mr. Crookes in the 

 Spiritualist, is no more effective in preventing the performance 

 of juggling tricks than Eva Fay's ordinary tying-down under 

 which her tricks were publicly reproduced two years ago by 

 Messrs. Maskelyne and Cooke. And since Mr. Crookes made 

 no mention of the extraordinarily sensitive galvanometer he 

 UFcd, v/hich is described for the first time by Mr. Wallace in 

 the last number of Eraser, I only consider myself bound to show 

 the method by which, with ordinary apparatus, the electric test 

 may be evaded — the trained skill of the acute trompcuse being 

 very probably required to meet the more severe test now first 

 specified. 



Mr. Wallace seems to me to have been a little hasty on another 

 point. " The supposed exposure of Eva l"ay in America," he 

 says, " was no exposure at all, but a clumsy imitation." As this 

 is merely Mr. W.'s dictum founded upon an imperfect newspaper 

 report, I prefer to trust the judgment of the eye-witnesses who 

 have publicly testified to the completeness of the exposure. Among 

 these are not only three of the ablest men in New York (the 

 Rev. Dr. Bellows, Ex-Surgeon-General Mott, and Dr. Ham- 

 mond), but the reporters of the very newspaper referred to which 

 had previously shown a decided leaning to the claims of spiri- 

 tualism. And their judgment is confirmed by the fact (which 

 Mr. Wallace probably considers as a newspaper fiction, but of 

 which I have mdependent testimony) that Eva Fay was forcca 

 by the local authorities to take out a licence as a juggler as a con- 

 dition of the continuance of her public performances. 



The fundamental difference between Mr. Wallace and myself 

 as to the validity of testimony in regard to the "occult" comes 

 out so strongly in this case that we have really no coinmon 



