ELECTROLYTES. 211 



a search for other membranes less susceptible to electrolytes. It is pro- 

 posed to give in the present chapter a brief account of our experience 

 with electrolytes during the early, or preliminary, stages of that in- 

 vestigation. 



Experiment 1. 



The first trial was with a 0.5 weight-normal solution of potassium 

 chloride. The cell selected was an unusually mature one. It had been 

 in use about four years, and had never failed, when properly treated, 

 to give a reliable measurement of the osmotic pressure of a non-elec- 

 trolyte. Through long use and frequent reinforcement, the membrane 

 had acquired a very high resistance, and the cell required a long time 

 for the establishment of equilibrium pressures. Nevertheless, because 

 of its proved reliability, it was still highly prized for the measurement 

 of the pressures of concentrated solutions, in which large thermometer 

 and barometer effects are of less relative importance than in dilute 

 solutions. The resistance of the cell at the time of setting it up with 

 the solution of potassium chloride was 1,170,000 ohms, and the temper- 

 ature of the bath was 30°. The initial pressure was adjusted to about 

 22.5 atmospheres. There followed some fluctuations of bath tempera- 

 ture and the mercury meniscus did not come to rest until the fourteenth 

 day. The indicated osmotic pressure of the solution at that time was 

 20.644 atmospheres. Twenty days later, it was 20.679 atmospheres. 

 The intermediate variations in pressure were small, and could be reason- 

 ably ascribed to thermometer and barometer effects. On the thirty- 

 fourth day — while still at full equilibrium pressure, and exhibiting no 

 signs of weakness — the cell was opened. The water in which the cell 

 had stood during the experiment was examined for chlorine. The 

 amount found was equivalent to 1.7 milligrams per 100 cubic centi- 

 meters. But, since the cell is always somewhat soiled by the solution 

 at the time of filling it, the presence of this small quantity of chlorine 

 was not believed to signify leakage on the part of the membrane. On 

 the whole, this determination of the osmotic pressure of the 0.5 weight- 

 normal solution of potassium chloride was, and still is, regarded as 

 probably very nearly correct. The mean of the osmotic pressures of 

 the first and last days of equilibrium is 20.662, while the mean of all 

 the 20 days of equilibrium is 20.610. The theoretical pressure, calcu- 

 lated — as best it may be — from Kohlrausch's values for the dissociation 

 of potassium chloride, and presuming no hydration of the solute to 

 exist which modifies the osmotic pressure, is 22.110 atmospheres. 



It was now to be determined, by means of a series of repetitions of the 

 experiment, whether the membrane had suffered, or does suffer, any 

 deterioration in consequence of contact with the electrolyte. Accord- 

 ingly the cell was soaked in water for six days, and then set up again 

 with another 0.5 weight-normal solution at the same temperature. 

 The resistance of the membrane on the second occasion was 400,000, 

 whereas it had been 1,170,000 ohms on the first trial. The cell re- 



