DIFFUSION. 633 



The coefficient of Intel-diffusion of two liquids must therefore be considered 

 as depending on all the physical properties of the mxiture according to laws 

 which can be ascertained only by experiment. 



Thus Fick has determined the coefficient of diffusion for common salt in 

 water to be O'OOOOOllB, and Voit has found that of cane-sugar to be 

 0-00000365. 



It appears from these numbers that in a vessel of the same size the 

 process of diffusion of liquids requires a greater number of days to reach a 

 given stage than the process of diffusion of gases in the same vessel requires 

 seconds. 



When we wish to mix two liquids, it is not sufficient to place them in 

 the same vessel, for if the vessel is, say, a metre in depth, the lighter liquid 

 will lie above the denser, and it will be many years before the mixture 

 becomes even sensibly uniform. We therefore stir the two liquids together, 

 that is to say, we move a solid body through the vessel, first one way, then 

 another, so as to make the liquid contents eddy about in as complicated a 

 manner as possible. The effect of this is that the two liquids, which originally 

 formed two thick horizontal layers, one above the other, are now disposed in 

 thin and excessively convoluted strata, which, if they could be spread . out, 

 would cover an immense area. The effect of the stirring is thus to increase 

 the area over which the process of diffusion can go on, and to diminish the 

 distance between the diffusing liquids ; and since the time required for diffusion 

 varies as the square of the thickness of the layers, it is evident that by a 

 moderate amount of stirring the process of mixture which would otherwise 

 require years may be completed in a few seconds. That the process is not 

 instantaneous is easily ascertained by observing that for some time after the 

 stirring the mixture appears full of streaks, which cause it to lose its trans- 

 parency. This arises from the different indices of refraction of different portions 

 of the mixture which have been brought near each other by stirring. The 

 surfaces of separation are so drawn out and convoluted, that the whole mass 

 has a woolly appearance, for no ray of light can pass through it without being 

 turned many times out of its path. 



Graham observed that the diffusion both of liquids and gases takes place 

 through porous solid bodies, such as plugs of plaster of Paris or plates of 

 pressed plumbago, at a rate not very much less than when no such body is 

 interposed, and this even when the solid partition is amply sufficient to check 



VOL. n. SO 



