124 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



The seeming adequacy of the interpretation of the loss in rotation 

 which is given above, and the attractive concordance which the results 

 of Series I and II acquired through its application were afterwards 

 proved to be wholly illusive. 



It was realized from the beginning that the diminished rotation 

 could also be produced by dilution. Indeed, this would have been the 

 most obvious interpretation of the phenomenon if the membranes had 

 failed to retain perfectly the solute. But, in the absence of leakage, 

 it was difficult to explain, as due to dilution, a loss in rotation which 

 amounted to an average of 2.86 per cent, or to an average surreptitious 

 introduction into the cells of nearly 0.5 cubic centimeter of the solvent. 



There were, nevertheless, three sources of dilution which were apparent 

 enough, but it was not believed that these could account for more than 

 a small fraction of the loss. However much their aggregate effect may 

 have been underestimated in the beginning, they were not at any time 

 ignored or neglected. It was recognized that, in order to settle definitely 

 the question of loss in rotation, all sources of dilution must be sup- 

 pressed by improvement in the method and in the manipulation. 



In discussing the three obvious sources of dilution which have been 

 referred to, it will be necessary to introduce observations and facts 

 which belong to later periods in the history of the investigation. If 

 this is not done, it will be difficult to place the results of Series II in the 

 light in which the author now sees them. 



1. It has already been intimated that the closing of the cell was a 

 difficult performance which required considerable time — in the begin- 

 ning, about 15 minutes. During the whole operation, the cell contents 

 were under a pressure which was less than the true osmotic pressure 

 of the solution. Throughout the whole of the closing period, therefore, 

 the solutions were undergoing a dilution by solvent taken in through 

 the membranes. If the impression is correct that the rate at which 

 the solvent is taken in, under such conditions, is proportional to the 

 difference between the existing and the true osmotic pressure of the 

 solution, the amount of dilution accomplished during the closing period 

 must have varied considerably from cell to cell. Operator "No. 1" 

 endeavored to maintain the highest possible "existing pressure" through- 

 out the operation, but was never able to equal the osmotic pressure. 

 Moreover, the pressure was constantly fluctuating in consequence of 

 the manipulations of "No. 2" with the "fang." 



It was evident that, in order to suppress this initial dilution of the 

 cell contents, "No. 1" and "No. 2" must cooperate in such a way as to 

 maintain the highest possible pressure upon the solution throughout 

 the closing period ; also, and above all, that the time required for closing 

 must be greatly shortened. The first improvement in the latter direc- 

 tion was accomplished by tightly wrapping and tying the lower end of 

 the rubber stopper — just above the enlargement on the manometer — 



