DIFFUSION. 635 



enabled to effect many remarkable analyses. He called this method the method 

 of Dialysis. 



Diffusion and Evaporation, Condensation, Solution, and Absorption. 



The rate of evaporation of liquids is determined principally by the rate 

 of diffusion of the vapour through the air or other gas which lies above the 

 liquid. Indeed, the coefficient of diffusion of the vapour of a- liquid through 

 air can be determined in a rough but easy manner by placing a little of the 

 liquid in a test tube, and observing the rate at which its weight diminishes 

 by evaporation day by day. For at the surface of the liquid the density of 

 the vapour is that corresponding to the temperature, whereas at the mouth 

 of the test tube the air is nearly pure. Hence, if p be the pressure of the 

 vapour corresponding to the temperature, and p = Jcp, and if ra be the mass 

 evaporated in time t, and diffused into the air through a distance h *, then 



j^ _ Jchm 

 ' pt' 



This method is not, of course, applicable to vapours which are rarer than 

 the superincumbent gas. 



The solution of a salt in a liquid goes on in the same way, and so does 

 the absorption of a gas by a liquid. 



These processes are all accelerated by currents, for the reason already ex- 

 plained. 



The processes of evaporation and condensation go on much more rapidly when 

 no air or other non-condensible gas is present. Hence the importance of the 

 air-pump in the steam engine. 



Relation between Diffusion of Matter and Diffusion of Heat. 



The same motion of agitation of the molecules of gases which causes two 

 gases to diffuse through each other also causes two portions of the same gas 

 to diffuse through each other, although we cannot observe this kind of diffusion, 

 because we cannot distinguish the molecules of one portion from those of the 



* h should be taken equal to the height of the tube above the surface of the liquid, together 

 with about f of the diameter of the tube. See Clerk Maxwell's Electricity, Art. 309. 



802 



