[lynde-dupkéT on osmosis IN SOILS 111 



To test the constancy of these osmotic pressures, we proceeded as 

 follows. After the osmotic pressures had become constant we allowed 

 the level of the distilled water to fall by evaporation. The solutions 

 fell also, but in all cases the osmotic pressures remained constant. 

 We then raised the level of the distilled water; the solutions rose also 

 and in a few days gave the former osmotic pressures. 



It will be noticed that in the case of the fine clay, a soil column 

 5-5 cms. long supports an osmotic pressure equivalent to a water 

 column 42 • 1 cm. long. That is, a water column nearly 8 times as high 

 as the soil is deep. 



This remarkable result led us to make experiments with soil 

 columns composed of still finer clay. For the soil columns we used 

 clay which had remained suspended in water for 1 week. With a soil 

 column 6 cms. deep we have obtained an osmotic pressure of over 

 315 grams per square centimeter. This pressure is equivalent to a 

 water column over a 315 cms. high, that is, a column over 50 times 

 as high as the soil is deep. The pressure is still increasing and the 

 final results will be given in a later paper. 



The efficiency of the soil constituents as semi-permeable membranes. 



To determine the efficiency of the soil constituents as semi-perme- 

 able membranes we proceeded as follows. 



(1) We compared the electrical resistance of the soil solutions 

 with that of a 0-0 2 N. K CI solution. 



(2) We calculated the osmotic pressure which each solution 

 should develop if the soil constituents were perfect semi-permeable 

 membranes. 



(3) From these and the observed osmotic pressures, we calculated 

 the efficiency of the soil constituents as semi-permeable membranes 

 for the solutions used. 



Method of Calculation. 



The molecular lowering of the freezing point of a KCl solution 

 containing 0*02 gram molecules per liter is 3-58° C. The molecular 

 lowering of the freezing point of a dextrose solution containing 0«0198 

 gram molecules per liter is 1'84°C. 



The abnormal effect of KCl upon the freezing point is due to the 

 fact that the KCl is dissociated in solution. A KCl solution contain- 

 ing 0-02 molecules per liter has the same effect as a dextrose solution 



containing 0-0198 X 3-58 , , 



= • 038 gram molecules per liter. 



1-84 "^ 



