1874.] caused by Evaporation and Condensation. 407 



The particles which impinge on the surface will rebound with a greater 

 velocity than that with which they approached ; 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 rebound with a less velocity than that at which they 

 approach. 



Mathematically the result may be expressed as follows the symbols 

 having the same meaning as before, e representing the energy communi- 

 cated in the form of heat, and $v the alteration which the velocity of the 

 molecule undergoes on impact. As before, 



and 



e = C 7T I ~ V =C -a nearl 7 



6 "2cj Qg 



j, dv * 



Therefore, in the case of steam at a temperature of 60, 



and in the case of air 



It must be remembered that e depends on the rate at which cold 

 particles will come up to the hot surface, which is very slow when 

 it depends only on the diffusion of the particles of the gas inter se and 

 the diffusion of the heat amongst them. 



It will be much increased by convection-currents ; but these will (as 

 has been already explained), to a certain extent, produce an opposite 

 effect. It would also seem that this action cannot have had much to do 

 with Mr. Crookes's experiments, as one can hardly conceive that much 

 heat could be communicated to the gas or vapour in such a perfect 

 vacuum as that he obtained, unless, indeed, the rate of diffusion varies 

 inversely as the density of a gas*. It will be interesting, however, to 

 see what light experiments will throw on the question. 



* June 10. Professor Maxwell has shown that the diffusion both of heat and of 

 the gas varies inversely as the density ; therefore, excepting for convection-currents, 

 the amount of heat communicated from a surface to a gas would be independent 

 of the density of the gas, and hence the force /would be independent of the density; 

 that is to say, this force would remain constant as the vacuum improved, while the 

 convection-currents and counteracting forces would gradually diminish. It seems 

 probable, therefore, that Mr. Crookes's results are, at least in part, due to this force. 



TOL. XXII. 2 I 



