THE MEMBRANES. 81 



water 10, 20, 40, and 60 grams of sugar; while those from whose pres- 

 sures the values given in column 4 were calculated contained in the 

 same volume of solvent 8.49, 16.98, 33.96, and 67.92 grams. They were, 

 in fact, 0.025, 0.050, 0.100, and 0.150 "weight -normal" solutions, whose 

 pressures were 0.64, 1.28, 2.54, and 4.99 atmospheres respectively. 



To one who has had long experience in the measurement of osmotic 

 pressure, the conformity of Pfeffer's results to the requirement of Boyle's 

 law — stated merely as proportionality of pressure to concentration — is 

 not surprising. It is, in fact, almost a necessary consequence of using 

 cells which do not quite perfectly retain the solute. The leakage of 

 solute from any series of defective, but equally good, cells is propor- 

 tional to the pressures of the solutions rather than to their supposed 

 concentrations. In other words, the pressures of cane-sugar solutions, 

 between 0° and 25°, are for some reason not proportional to their sup- 

 posed concentration. If now a series of them of varying strength are 

 placed in defective cells, the escape of solute will be proportional to the 

 pressures, and not to the supposed relative concentration of the solu- 

 tions. The necessary consequence is that the relative pressures will 

 gradually approach proportionality to the (supposed) relative concen- 

 trations of the solutions. In a later chapter the author will have occa- 

 sion to show how membranes which do not leak may be the means of 

 appearing to establish the applicability of Boyle's law to osmotic pres- 

 sure upon evidence which is superficially convincing, but fundamentally 

 unsound. It may be stated here that all solutions of cane sugar thus 

 far investigated do obey the law of Gay-Lussac between 0° and 25°, 

 while none of them appear to obey the law of Boyle until higher temper- 

 atures are reached. That this failure, in the latter case, may be more 

 apparent than real will be shown elsewhere. 



It is fortunate that the validity of the generalizations of van't Hoff 

 regarding solutions does not depend exclusively upon the correctness of 

 Pfeffer's measurements; and, in one sense, it is also fortunate that 

 Pfeffer obtained the results which he did, rather than the correct pres- 

 sures of his solutions; for, whatever may have been the relative impor- 

 tance assigned to them by van't Hoff, they undoubtedly contributed 

 more than any other one of his arguments to the immediate and general 

 acceptance of his views. 



The first announcement* regarding the electrolytic method of deposit- 

 ing membranes was made in the following words : 



"If a solution of a copper salt and one of potassium ferrocyanide are sepa- 

 rated by a porous wall which is filled with water, and a current is passed from 

 an electrode in the former to another electrode in the latter solution, the copper 

 and the ferrocyanogen ions should meet within the wall and separate as copper 

 ferrocyanide at all points of meeting, so that in the end there should be built 

 up a continuous membrane well supported on either side by the material of 

 the wall." 



*Amer. Chem. Journal, xxvi, 81. 



