92 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



The same is probably true of a number of the cobalticyanides, but the 

 experimental evidence with regard to the last class of salts is still too 

 meager to warrant any positive statements concerning their durability. 

 A ferrocyanide of copper membrane can be satisfactorily reinforced 

 with the nickel or the cobalt salt, and vice versa. 



6. THE EFFECT OF ELECTROLYTES. 



It has been shown that the effect on the membranes of the alkali 

 which is liberated during their deposition is undoubtedly injurious; 

 and the impossibility of building up good membranes and of main- 

 taining them thereafter in good condition, without repeated and pro- 

 longed soaking in pure water, has been ascribed to the accumulation 

 of potassium in the membranes. It was therefore apprehended that the 

 measurement of the osmotic pressure of potassium salts, and perhaps 

 of other electrolytes, would be attended with great difficulties. These 

 fears have been partially, but not fully, realized. Our experience with 

 the electrolytes will be given in Chapter XI, which describes our efforts 

 to measure the osmotic pressures of potassium and lithium salts. 



7. THE SEMIPERMEABILITY OF MEMBRANES. 



It is often asserted that no membrane is truly semipermeable; in 

 other words, it is frequently affirmed that all membranes are perme- 

 able to the solute as well as to the solvent — that the difference is one 

 of degree only. Such statements appear to be without justification. 

 They are certainly not founded on any reliable information in the 

 possession of those who make them. The question is one of funda- 

 mental importance and is to be decided only by experiment. The 

 fact that all the solutions of cane sugar and glucose which are taken 

 from good cells, after a measurement of osmotic pressure, are found 

 to have maintained their concentration perfectly, is evidence enough 

 that membranes may be made sufficiently semipermeable for quanti- 

 tative purposes. But, though many of our cells had maintained, with- 

 out evidence of weakness, the maximum pressures of the solutions for 

 10, 15, and even 20 days, it was decided to test the soundness of the 

 membranes by means of an experiment of much longer duration. For 

 this purpose, a cell containing a 0.5 weight-normal solution of cane 

 sugar was selected at random and allowed to remain in the bath at 

 15° for two full months. The record of the cell is given in Table 5 in 

 atmospheres of osmotic pressure. 



At the end of the two months, the solution was removed from the 

 cell and compared in the polariscope with a reserved portion of the 

 original solution. The rotations of the two were identical, showing 

 that no leakage of the solute had occurred. 



The cell employed in this experiment is a good example of what 

 came to be known as " quick cells." In less than 24 hours after setting 



