82 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



The electrolytic method of depositing membranes was soon success- 

 fully applied to all the usual varieties of porous vessel which accumulate 

 in a laboratory, and to several new forms of the same. It was found, 

 in fact, that a highly active membrane could be produced without diffi- 

 culty in every kind of porous wall. The method was also employed 

 for the deposition in porous vessels of many other compounds than 

 copper ferrocyanide, and a large number of these (about 25) proved to 

 be osmotically active. 



The success of the new method as a means of building up membranes 

 was placed beyond question, but the possession of it did not enable us to 

 proceed at once to the measurement of osmotic pressure. The resisting 

 power of the membranes made by the electrolytic method was much 

 greater than that of those produced by the process of Pfeffer, but even 

 they were unable, in the porous vessels then available, to withstand high 

 pressures without rupture. Our attention during the succeeding four 

 years was given almost exclusively to the porous wall on, or within, 

 which the membrane is deposited. 



A brief account of this part of the investigation has been given in the 

 first chapter; but a concise restatement of those structural character- 

 istics of the cell wall, upon which the efficiency of the membrane was 

 found to depend, will be useful in this place. They are : 



1. A great and uniform strength of wall, which was secured by 

 employing mixtures of different clays. 



2. Absence of "air blisters," which were eliminated by subjecting 

 the clays to great pressures. 



3. An exceedingly fine and uniform porosity, which was secured by 

 the employment of the finest portions only of the clays, by 

 subjecting the mixtures to high pressure, and by burning at 

 high temperatures. 



THE DEPOSITION OF THE MEMBRANE. 



The first steps toward the formation of the membrane are the expul- 

 sion of air from the pores of the cell and its replacement by water. This 

 has been effected from the beginning by means of the considerable vol- 

 ume of water transported by the cations whenever dilute aqueous solu- 

 tions of salts are subjected to electrolysis. At first the salt employed 

 was potassium sulphate, but the fact that the "atmosphere" of water 

 which surrounds the lithium ion is much greater than that transported 

 by the potassium ion suggested the use of lithium salts rather than 

 those of potassium. A series of quantitative comparisons carried out 

 by Frazer showed that the quantities of water drawn through the cell 

 wall conform to the following rule: 



The volumes of water carried through the porous wall of a cell, under 

 identical conditions, are inversely proportional to the relative velocities of 

 r ,~v the various cations, divided by their respective valencies. 



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