ELECTROLYTES. 213 



The supposed "protective" action of such colloids as albumen and 

 gelatin was also tested with the chlorides of potassium and barium, but 

 without discoverable advantage to the membranes. 



One experiment was carried through with potassium ferrocyanide, 

 but the membrane gave the same unmistakable evidence of deteriora- 

 tion under the influence of this electrolyte that it had exhibited when 

 tested with potassium chloride. 



A few experiments were made with potassium chloride in cells having 

 membranes of nickel ferrocyanide. Membranes of this material are 

 probably somewhat superior to those of copper ferrocyanide for the 

 measurement of the pressures of non-electrolytes. They were found, 

 however, to have no advantage over the latter for use with electrolytes. 



Some evidence was gathered to the effect that it will be possible to 

 measure the osmotic pressure of quite dilute solutions of potassium 

 salts, even with the copper ferrocyanide membrane. This is of interest 

 in connection with the fact, as will be shown later, that the practicability 

 of measuring the osmotic pressure of lithium salts is altogether a ques- 

 tion of concentration. 



When cells of demonstrated excellence were set up with half normal 

 solutions of potassium chloride, there was always obtained on the first 

 trial a high pressure. On one occasion, it was probably the maximum 

 osmotic pressure of the solution. In all succeeding trials, however, 

 smaller and smaller pressures were obtained, until, in some instances, 

 the pressure observed on the third trial had fallen to about one-half of 

 its first value. Such conduct on the part of the cells can only be 

 explained by supposing that the membranes had degenerated to the 

 point of becoming quite permeable to the solute, and one would expect, 

 perhaps, to find considerable chlorine in the water in which the cells 

 had stood. As a matter of fact, however, the amount of it which made 

 its way into the water surrounding the cells was very small, and in no 

 instance sufficient to account for more than a minute fraction of the 

 deficit in pressure. This observation is cited here in order to empha- 

 size again the fact — already more than once stated — that it is useless 

 to attempt to measure osmotic pressure in leaky cells; because the 

 escaped solute always concentrates heavily in the pores of the cell wall, 

 giving a solution of wholly unknown concentration in contact with the 

 exterior surface of the membrane. Such concentration may be due in 

 part to lack of time for diffusion, but it is probably due in much greater 

 measure to adsorption. 



In general, high resistance is regarded as a good sign in a membrane; 

 but it is certainly no proof of its ability to measure osmotic pressure, 

 if the membrane has once suffered injury from contact with electrolytes. 

 In some later experiments with potassium chloride, where the cells 

 were unable to develop even half the normal pressures of the solutions, 

 the membranes had still a resistance of more than a half million ohms. 



