caused by Evaporation and Condensation. 149 



passing off into the gas, it must be shot off with a velocity greater 

 than that of a cannon-ball. Whatever may be the nature of the 

 forces which give it the velocity, and which consume the latent 

 heat in doing so, it is certain, from the principle of conservation 

 of momentum, that they must react on the surface with a force 

 equal to that exerted on the molecule, just as in a gun the pressure 

 of the powder on the breech is the same as on the shot. 



The impulse on the surface from each molecule which is driven 

 off by evaporation must therefore be equal to that caused by the 

 rebound of one of the reflected molecules, supposing all the mo- 

 lecules to be of the same size ; that is to say, since the force of 

 rebound will be equal to that of stopping, the impulse from a par- 

 ticle driven off by evaporation will be half the impulse received 

 from the stopping and reflection of a particle of the gas. Thus 

 the effect of evaporation will be to increase the number of impulses 

 on the surface ; and although each of the new impulses will only be 

 half as effective as the ordinary ones, they will add to the pressure. 



In the same way, whenever a molecule of gas comes up to a 

 surface and, instead of rebounding, is caught and retained by the 

 surface, and is thus condensed into a molecule of liquid, the impulse 

 which it will thus impart to the surface will only be one half as 

 great as if it had rebounded. 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 pres- 

 sure would be greater on the moist surface than on that which 

 was dry. And, again, if one of the surfaces were 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, 



j> 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 



__dv 



""6"' 



and the pressure jp will be given by 



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

 * See Maxwell, 'Theory of Heat,' p. 294. 



