84 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



during the electrolysis, the ferrocyanide is renewed every 2 or 3 min- 

 utes by pouring a fresh solution of it into the funnel. A temporary 

 increase in resistance follows each renewal, but this may be due in part 

 to a fall in the temperature of the contents of the cell. The final decline 

 in resistance may be postponed by frequent renewals of the solution, but 

 not indefinitely. For this reason, it is suspected that the phenomenon 

 is possibly due to accumulation of alkali in the membrane. Having 

 reached its maximum resistance, the membrane can not be further 

 improved by electrolysis, without a thorough preliminary soaking in 

 pure water — that is, water which is free from electrolytes. The cell is 

 therefore placed in water in which a little thymol has been dissolved, 

 and is allowed to soak, with frequent renewals of the water, for several 

 days. The period of soaking is quite indefinite, but experience has 

 shown that it should be not less than 3 days; and that, in general, 

 the longer the soaking is the better will be the result of the succeeding 

 electrolysis. 



After soaking, not less than 3 days, the membrane-forming process is 

 repeated. On this occasion the resistance usually rises much higher 

 than before, but finally reaches a maximum beyond which it can not be 

 driven, however frequently the solution of ferrocyanide may be renewed. 

 If the electrolysis is continued beyond this point, a gradual fall in resist- 

 ance sets in, which may eventually lead to the total ruin of the mem- 

 brane. As in the first instance, when it is found that the resistance no 

 longer increases, the electrolysis is interrupted and the cell is again 

 placed in water for a period of 3 or more days. The further procedure 

 is simply a repetition of the alternate "running" and "soaking" described 

 above, which is persisted in until the resistance of the membrane can be 

 forced no higher. The maximum resistance which is finally obtained 

 varies greatly. Obviously, it varies with the effective area of the 

 membrane; and this, in turn, varies from cell to cell, according to the 

 porosity of the cell wall. In general, it is found that membranes in 

 hard-burned cells have high final resistances. The temperature of depo- 

 sition is also an important factor in determining resistance. In a given 

 cell, a membrane deposited or repaired at 0° may have a resistance of 

 more than 1,000,000 ohms, while at 80° the resistance can not be driven 

 above 1,000 ohms. Again, the resistance of the membranes increases 

 with age and repeated use. 



Having developed the maximum resistance in the manner described, 

 the membrane is subjected to a process of "seasoning" under pressure. 

 For this purpose the cell is set up with a concentrated solution of cane 

 sugar (not less than half normal) to which a small amount of ferro- 

 cyanide has been added, an osmotically equivalent quantity of copper 

 sulphate having been dissolved in the water in which the cell is to 

 stand during the experiment. The initial mechanical pressure which is 

 brought upon the contents of the cell at the time of closing may be 



