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LIII. On the Surface-Forces caused by the Communication of Heat. 

 By Professor Osborne Reynolds*. 



IN a paper read before the Royal Society, June 18, I pointed 

 out, as it seemed to me, that whenever evaporation or con- 

 densation takes place on a surface they are attended with certain 

 forces tending respectively to drive the surface back and urge it 

 forward, these forces arising, according to the kinetic theory, from 

 the momentum which is imparted from the surface to the particles 

 driven off, and vice versa. I also pointed out at the end of the 

 paper that similar effects will be produced whenever heat is com- 

 municated from a surface to a gas, and vice versa. The possibility 

 of this latter effect only occurred to me as I was on the point of 

 sending off the paper, and consequently was added by way of an 

 appendix. The first part of the paper contains a description of 

 some experiments undertaken to verify my conclusions respecting 

 the forces of evaporation and condensation, the results of which 

 seem to me to be fully explained by these forces; so that had I 

 rewritten the paper after becoming aware of the possible existence 

 of the other force, I should have had nothing to add in connexion 

 with these experiments. I had, however, also endeavoured to 

 show that the first class of forces afforded an explanation of Mr. 

 Crookes's experiments ; and had this part of the paper been re- 

 written it would have been somewhat altered, as the last class of 

 forces (those arising from the simple communication of heat) 

 seem to afford a simpler explanation of some of the phenomena 

 observed by Mr. Crookes. I regret that this was not done, as, 

 from some remarks in a paper published in the August Num- 

 ber of the Philosophical Magazine, I fear that Mr. Crookes has 

 not understood my meaning, and has consequently been at the 

 trouble of making further experiments, which, however valuable 

 from other considerations, throw no fresh light on the case in 

 point. However, before proceeding to discuss the subject fur- 

 ther, I would set myself straight with Mr. Crookes in one or two 

 particulars. 



Mr. Crookes appears to complain that I did not give him 

 credit for having obtained evidence of repulsion by heat in a 

 medium as dense as that which I used, viz. from i to j inch of 

 mercury. Now the only account of his experiments which I had 

 seen was the abstract published in the i Proceedings of the Royal 

 Society/ December 1 ; and in this the highest pressure at which 

 he definitely states he obtained repulsion is 3 millimetres, or one 

 tenth of an inch : but this in truth was not the point. In art. 

 44 of his paper he describes an experiment in which he did not 



* Communicated by the Author. 



