DIFFUSION OF SALTS 193 



solution is maintained unaltered at one end of a cylinder 

 of water, the total quantity of salt which diffuses, and the 

 distance travelled by the salt, are both proportional to the 

 square root of the time of diffusion, so long as any pure 

 water remains in the cylinder. Thus the amount of salt 

 diffusing in twenty-five hours would be only five times the 

 amount diffusing in the first hour. The case here assumed 

 would practically occur when any top dressing of a soluble 

 salt is applied to a wet soil, and would continue so long 

 as a saturated solution of that salt remained at the surface. 

 As soon however as the concentration of the diffusing salt 

 solution is diminished the rate of diffusion will considerably 

 decrease. 



Within certain limits, the rate of diffusion is proportional 

 to the strength of the salt solution, a solution of twice the 

 strength diffusing twice as much in the same time. 



Though a salt diffuses into a weak solution of the same 

 salt much more slowly than into pure water, it will diffuse 

 into a solution of another salt with nearly the same speed 

 as into pure water. This fact is of great importance, and 

 much increases the practical effects arising from diffusion. 

 If in any part of a moist soil a salt is removed from solution 

 by the action of roots, or by the chemical absorption of the soil, 

 a flow of that salt will take place at once towards the spot, 

 irrespective of the quantity of other salts which may be present. 

 Diffusion is considerably influenced by temperature, and 

 becomes more rapid as the temperature rises. Thus Graham 

 found that when the temperature of the solution was 67 F. 

 chloride of sodium diffused rather more than one-third more 

 rapidly than when its temperature was 39-6. 



Different substances have very different rates of diffusion. 



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