198 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



The determinations of the osmotic pressures of cane sugar and glucose 

 presented in Chapters VIII and IX were designated as "final" in order 

 to express the confidence of the author in the general correctness of 

 the results. The measurements contained in the present chapter are 

 not so designated, because one essential test of their reliability has 

 not been applied to them: It was not proved that the solutions of 

 mannite maintained perfectly their concentration while in the cells. 

 It was easy to do this in the case of cane sugar and glucose, because 

 slight changes in the concentration of solutions could be detected and 

 measured by the polariscope; but in that of mannite, there was at the 

 time no convenient analytical method available. The ' ' interferometer " 

 made by Zeiss has since been introduced for use with optically inactive 

 substances, and will be employed when the "final" determinations of 

 the osmotic pressure of mannite are undertaken. The author's reasons 

 for insisting on a perfect maintenance of concentration while the solu- 

 tions are in the cells, as an indispensable part of the evidence of credi- 

 bility, have already been given, and they still seem to him perfectly 

 valid, and worthy of the strongest emphasis. Nevertheless, he does 

 not wish to be understood as intimating that any great amount of 

 suspicion attaches to the present determinations of the osmotic pressure 

 of mannite, because of the absence of this proof. On the contrary, he 

 believes the results to be quite trustworthy. 



A large proportion of the cells which were used with mannite had 

 previously been employed in measuring the osmotic pressure of lithium 

 chloride. The effect of exposure to an electrolyte is to render the 

 membranes sluggish. Evidence of that result is probably to be seen 



(1) in the long time consumed by the cells in coming to equilibrium, and 



(2) in the considerable thermometer effects, i. e., in the rather large fluc- 

 tuations in pressure from day to day. As usual when membranes of 

 diminished activity are employed, the cells were generally allowed to 

 make long records for the purpose of minimizing thermometer effects. 

 It is, however, not certain, in this instance, that the remarkable tardi- 

 ness of the cells in coming to equilibrium was due wholly to the effect 

 of the electrolyte upon the membranes; for it was observed that certain 

 cells, whose membranes had not been in contact with an electrolyte, 

 were likewise very slow in establishing their final pressures. It has 

 not yet been determined whether this was due to the hard burning of 

 the cells — in effect to the limited area of the membranes — or to some 

 peculiarity of the mannite which distinguishes it from cane sugar and 

 glucose, or other non-electrolytes. Hitherto, we have had no reason 

 to suspect, in the case of non-electrolytes, that the activity of the mem- 

 brane varies with the solute. The question is an important one, and 

 it will be carefully investigated. 



