390 On the Surface-Forces caused by the Communication of Heat, 



obtain repulsion until the Sprengel pump had been at work for 

 a long time after the gauge showed half a millimetre. It was 

 the results of this experiment which I was endeavouring to ex- 

 plain, and consequently it was to this experiment that my remarks 

 applied ; and I had not the least intention of implying that these 

 were the only results which Mr. Crookes obtained. However, 

 had it not been so, had I misread Mr. Crookes's paper as he 

 supposed, I think that he would have forgiven me when he sees 

 that he has committed a similar offence against me. He com- 

 mences his remarks on my paper by saying, " In my exhausted 

 receiver he assumes the presence of aqueous vapour ;" whereas 

 nowhere in my paper do I mention any such assumption, nor did 

 it enter into my head to make it. Nay, further, I think I have 

 shown, however darkly, that, under the conditions under which 

 Mr. Crookes's experiments were made, aqueous vapour would 

 not be sufficient to explain the results, since it would be to all 

 intents a non-condensable gas. However, enough of this. 

 So far as I can see, the case now stands thus : — 



1. Whenever a body is surrounded by a condensable medium 

 (that is, vapour at its point of saturation), heating or cooling of 

 the body will be respectively attended with evaporation and con- 

 densation, and hence with forces over the surface which changes 

 temperature. 



2. The amount of evaporation or condensation will not depend 

 on the density of the vapour with which the surface is surrounded, 

 provided only that it be at its point of saturation, but will depend 

 on the amount of heat available ; that is to say, it will depend 

 on the amount of heat imparted to or taken from the body. 

 Thus the evaporation of mercury would take place as readily in 

 a medium of too small density to be measured as the evapora- 

 tion of water under the pressure off of an inch. 



3. The presence of a non- condensable gas will greatly retard 

 the rate of evaporation and condensation. 



4. That under the conditions (1), there will be forces arising 

 from convection-currents in the surrounding medium, which 

 will generally act in opposition to the forces (1), but which will 

 diminish with the density of the medium, while the other forces 

 remain constant and therefore must ultimately prevail. 



5. That there is yet another set of forces, which act when the 

 medium is not in a state of saturation, i. e. is not condensable. 

 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 communicated ; and 

 since this rate has been shown by Professor Maxwell to be inde- 

 pendent of the density of the gas, these forces, like those arising 

 from condensation and evaporation, will be independent of the 



