112 THE ROYAL SOCIETY OF CANADA 



Note. — It must be noted here that we determined the resistance 

 of a 0'02 A^. KCl solution, that is, of a solution in which 0-02 gram 

 molecules of KCl are dissolved in 1000 cc. of solution, whereas the 

 molecular lowering of the freezing point given above, was determined for 

 a solution containing 0-02 gram molecules of KCl in 1000 cc of water. 

 This introduces a slight error. But an assumption which we make 

 later, namely, that the salts in the soil solution are dissociated to the 

 same extent as the 0-02 A^ solution of KCl, probably introduces a 

 greater error. We, therefore, cannot claim that the results of these 

 calculations are more than an approximation. 



Since the freezing point lowering and the osmotic pressure of a 

 solution are both proportional to the total number of particles (mole- 

 cules and ions) in a given volume of the solution, a 0-02 JV. KCl solu- 

 tion produces the same osmotic pressure as a dextrose solution con- 

 taining 0-038 gram molecules per liter. 



One gram molecule of dextrose dissolved in 22-4 liters of water 

 at 0° C. produces an osmotic pressure of 1 atmosphere or 1033' 6 

 grams per square cm. Therefore «038 gram-molecules in 1 liter of 

 water at' 16° C. produces an osmotic pressure of: — 



1033-6 X -038 X 22-4 X 289 

 = 931-4 grams per square cm. 



^ i Ô 



This is the pressure which should be developed at 16° C. by -038 

 gram-molecules of sugar in 1 liter of water or by a 0-02 N. KCl sol- 

 ution if the semi-permeable membrane were perfect. 



Let us assume that the salts in the soil solutions are dissociated 

 to the same extent that the 0-02 A^. KCl solution is. We can then 

 calculate from their electrical resistances the osmotic pressure that 

 these solutions should develop with a perfect semi-permeable mem- 

 brane, and then the efficiency of the soil columns as semi-permeable 

 membranes. 



Example. — From Table 3 below we learn that the solution of 

 clay (1) has a resistance of 1400 ohms and 0-02 A^ KCl a resistance 

 of 250 ohms at 16° C. 



If the soil were a perfect semi-pei'meable membrane, the solution 

 should develop a pressure of: — 



250 

 931-4 X-— — = 166-3 grams per sq. cm. 



The osmotic pressure observed is 4-4 grams per square cm. 

 Therefore the efficiency of clay (1) as a semi-permeable membrane is 



4-4 X 100 



