404 Prof. O. Reynolds on Surface-Forces [June 18, 



Hence condensation will reduce the magnitude of some of the impulses, 

 and therefore will reduce the pressure on the condensing surface. 



For instance, if there were two surfaces in the same vapour, one of 

 which was dry and the other evaporating, then the pressure would 

 be greater on the moist surface than on that which was dry. And, 

 again, if one of the surfaces was dry and the other condensing, then 

 the pressure would be greater on the dry surface than on that which 

 was condensing. Hence, if the opposite sides of a pith-ball in vapour were 

 in such different conditions, the ball would be forced towards the colder 

 side. 



These effects may be expressed more definitely as follows : 

 Let v be the velocity with which the molecules of the vapour move, 

 p the pressure on a unit of surface, 

 d the weight of a unit of volume of the vapour, 

 w the weight of liquid evaporated or condensed in a second ; 

 then the weight of vapour which actually strikes the unit of dry 

 surface in a second will be 



.* 



"IP 



and the pressure p will be given by 



*= 2 |' *> 



and / (the force arising from evaporation) will be given by 



fiSi 



9 

 therefore 





Thus we have an expression for the force in terms of the quantity of 

 water evaporated and the ratio of the pressure to the density of the 

 vapour; and if the heat necessary to evaporate the liquid (the latent 

 heat) is known, we can find the force which would result from a given 

 expenditure of heat. 



Applying these results to steam, we find that, at a temperature of 60, 

 the evaporation of 1 Ib. of water from a surface would be sufficient to 

 maintain a force of 65 Ibs. for one second. 



It is also important to notice that this force will be proportional to the 

 square root of the absolute temperature, and, consequently, will be 

 approximately constant between temperatures of 32 and 212. 



If we take mercury instead of water, we find that the force is only 

 6 Ibs. instead of 65 Ibs. ; but the latent heat of mercury is only ^ that of 

 water, so that the same expenditure of heat would maintain nearly three 

 times as great a force. 



It seems, therefore, that in this way we can give a satisfactory ex- 

 * See Maxwell, ' Theory of Heat,' p. 291. 



