192 MOVEMENTS OF SALTS IN THE SOIL 



water, and partly by the actual movements of water within 

 the soil. The greatest movements of salts are occasioned 

 by the latter cause, but the influence of the former is by 

 no means insignificant. 



Diffusion of Salts. If a solution of any salt is placed in 

 contact with pure water the salt at once commences to dis- 

 tribute itself throughout the water, and, if sufficient time 

 is allowed, the whole mass of water becomes at last a solution 

 of uniform composition. This movement of the molecules of 

 salt in water is akin to the self-distribution of the molecules 

 of a gas in air ; it will occur in the absence of all movement 

 in the water, and is indeed best studied by experimenting 

 with masses of solid jelly, in which the diffusion of salts is 

 quite as active as in fluid water. 



The rapidity of diffusion depends on the difference in the 

 concentration of the salt in adjacent parts of the mass of 

 water ; the greater the difference the more rapid is the dif- 

 fusion. Thus salt will diffuse more rapidly into water from 

 a strong solution than from a weak ; and more rapidly into 

 pure water than into a weaker solution of the same salt. If 

 we suppose a saturated solution of a salt placed at the bottom 

 of a vessel of pure water, we have at starting a maximum 

 rate of diffusion. Presently, however, the strong solution at 

 the bottom is diffusing into a weaker solution of salt lying 

 above it, and the pure water in the upper part of the vessel 

 is in contact, not with a strong, but with an extremely weak 

 solution of salt lying under it. The speed with which the 

 salt spreads through the water will thus rapidly diminish, 

 and in the final stages of the diffusion process becomes 

 extremely slow. 



For cases in which the concentration of the diffusing salt 



