66 



THE WATER-SUPPLYING POTV^R OF THE SOIL 



the rates of the third test. The essential data for such a consideration 

 are given in table 2, where the last column is devoted to the approxi- 

 mated increments in hourly rate, per degree rise in temperature. With 

 the exception of the first hour, during which it may be assumed that 

 adjustments of the membrane were in progress, it is apparent that each 

 rise of one degree in temperature is accompanied by an increase in 

 absolution rate of about 0.04 c.c. per hour, per 10 sq. cm. of active sur- 

 face. It is clear at once that this increment value is far too large to be 

 primarily related to the simple operation of the principle of Gay- 

 Lussac as applied to the diffusion tension of the solute in a solution. 

 If the higher rates shown at higher temperatures were simply related 

 to the temperature effect on the diffusion tension of the sugar, we 

 should expect an increment equal to about ^^ of the rate observed 

 with the lower temperature range. Such an increment would have 



Table 2. — Osmometer A, operating against water. Means of data from first and 

 second tests, compared with data from third test. 



only about one-tenth of the magnitude here experimentallj^ indicated. 

 It appears possible that temperature alterations in the state of the 

 colloidal membrane may contribute something to the great tempera- 

 ture variation in question, but no information is available on this point. 

 Decrease in the viscosity of the sugar solution with increasing tem- 

 perature, however, appears to be much more significant than the 

 features just mentioned, and this is, perhaps, the major factor in pro- 

 ducing the variations in the rate of water intake of our osmometers. 

 This rate is conditioned by the degree of concentration of the laj-er of 

 solution next the membrane. The factors tending to maintain this 

 at constant value are the gravitational downward movement of the 

 sugar solution distant from the membrane and the corresponding rise 

 along the membrane surface, the diffusion of the sugar molecules into 

 the diluted region next the membrane, and, opposed to these, the diffu- 

 sion of the sugar outward through the membrane. Increase in tem- 

 perature should increase diffusion into the diluted region and outward 

 from the osmometer. However, as is well known, the diffusion of 

 cane-sugar from concentrated to weak solutions is extremely small for 



