58 THE WATER-SUPPLYING POAVER OF THE SOIL 



One of the most serious difficulties met with in osmometric experimen- 

 tation arises from the fact that entrance of water through the osmotic 

 membrane into the solution produces, ipso facto, a dilution of the 

 latter, a dilution which, when of considerable amount, can not be 

 readily dealt with mathematically on account of the non-uniformity 

 of dilution throughout the mass of solution. In our studies it was 

 necessary that water be allowed continously to enter the osmotic 

 solution, for the rate of such entrance was just what it was desired to 

 measure. The immediate effect of such entrance should be, of course, 

 to dilute the layer of sugar solution lying against the membrane, thus 

 decreasing the attraction of the instrument for water and seriously 

 complicating the results. The total volume of water entering the 

 osmometer during a test was always exceedingly small, however, as 

 compared to the volume of the entire contents. It appears, then, that 

 if the solution in the cell might be mechanically stirred during the opera- 

 tion of the instrument, the dilution in question would probably be 

 quite negligible, at least in such crude measurements as are here con- 

 templated. No mechanical method for stirring the solution was tried, 

 although one may, no doubt, be somewhat readily devised. 



In our earlier experiments the thistle-tube osmometers were erected 

 vertically in water, with the collodion membrane horizontal and below 

 the mass of sugar solution in the bulb. It was thought that this 

 arrangement might promote a continuous convection current in the 

 solution, which might act as a stirrer. Tests indicated that such con- 

 vection did indeed take place, the more concentrated (heavier) solution 

 apparently passing downward along the glass walls and flowing out 

 upon the collodion film below, thus forcing the more dilute (lighter) 

 solution lying against the membrane to pass toward the center of the 

 latter and thence upward. But the viscosity of the sugar solution used 

 was so great and the form of the thistle bulb is so poorly adapted to the 

 maintenance of uniform convection within, that the current set up was 

 far from continuous. Rather did it appear that the heavier and more 

 concentrated solution from above moved downward only spasmodically ; 

 it slumped down in relatively large masses, at somewhat regular inter- 

 vals, and between the periodic slumps the whole contents of the bulb 

 seemed to remain practically static, excepting for the gradual entrance 

 of water through the membrane below. This condition of affairs was 

 clearly indicated by a curious periodic fluctuation in the rate of water 

 entrance into osmometers thus arranged. An initially high rate of 

 intake gradually fell during a time period and then suddenly increased 

 to a value approximately the original one, after which a decrease again 

 occurred, the variation in rate being thus cyclically continued. 



These disconcerting fluctuations in the rate of water intake were at 

 length avoided, simply by altering the position of the osmometer 



