1874.] caused by Evaporation and Condensation. 403 



be insensible, except when the heat on the surface was sufficiently- 

 intense to drive the vapour off with considerable velocity. This, indeed, 

 might be the case if vapour had no particles and was, what it appears to 

 be, a homogeneous ,elastic medium, and if, in changing from liquid into 

 gas, the expansion took place gradually, so that the only velocity acquired 

 by the vapour was that necessary to allow its replacing that which it 

 forces before it and giving place to that which follows. 



But, although it appears to have escaped notice so far, it follows, as a 

 direct consequence of the kinetic theory of gases, that, whenever evapo- 

 ration takes place from the surface of a solid body or a liquid, it must 

 be attended with a reactiquary force equivalent to an increase of pressure 

 on the surface, which force is quite independent of the perceptible 

 motion of the vapour. Also, condensation must be attended with a force 

 equivalent to a diminution of the gaseous pressure over the condensing 

 surface, and likewise independent of the visible motion of the vapour. 

 This may be shown to be the case as follows : 



According to the kinetic theory, the molecules which constitute the 

 gas are in rapid motion, and the pressure which the gas exerts against 

 the bounding surfaces is due to the successive impulses of these molecules, 

 whose course directs them against the surface, from which they rebound 

 with unimpaired velocity. According to this theory, therefore, whenever 

 a molecule of liquid leaves the surface henceforth to become a molecule 

 of gas, it must leave it with a velocity equal to that with which the 

 other particles of gas rebound that is to say, instead of being just 

 detached and quietly 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 molecules 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 particle 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. 



