78 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



osmotic phenomena, but when they were tested as to their sufficiency 

 for quantitative purposes by the rule that a perfect membrane must be 

 able to develop and maintain maximum pressures, without leakage of 

 the solute at any time during an experiment, they were found to be 

 wanting. 



The failure of the membranes to justify the hopes which had been 

 entertained of them could be ascribed to any one, or all, of several 

 causes — to the want of perfect semi-permeability on the part of the 

 copper f errocyanide ; to the faulty character of the supporting porous 

 wall; or to the imperfect attachment of the membrane to the wall; or to 

 all three of these possible defects. It was strongly suspected that the 

 third cause for failure existed; in other words, that the membranes made 

 by the method of Pfeffer are not at all points firmly attached to the 

 supporting wall. 



£ The ideal membrane (as regards location) is obviously one consisting 

 of innumerable plugs which are driven so firmly into the mouths of the 

 pores which open on the interior surface of the cell that no pressure can 

 rupture or dislodge them — that is, the membrane must be firmly 

 embedded in the wall and not consist of a mere (more or less detached) 

 cover for its interior surface. Looked at from this point of view, the 

 practice of Pfeffer of carefully drying the interior of his cells with filter 

 paper was a highly rational procedure. Subsequent observations have 

 proved it to be the vital feature of his process. Pfeffer himself recog- 

 nized this in a practical way, but he appears to have regarded these 

 membranes as u aufgelagert" rather than embedded, though he gives no 

 explanations; for he says (page 9), "wahrend ich anfangs mit grossen 

 Schwierigkeiten zu kdmpfenhatte, und ehe ich.zu partieller Abtrocknung 

 meine Zuflucht nahm, uberhaupt keine aufgelagerte Membran zu Stande 

 brachte." The obvious purpose of the u Abtrocknung" was to empty the 

 mouths of the pores of the solution of copper sulphate, in order that they 

 might be filled with the solution of potassium ferrocyanide and thus 

 force the formation of an embedded membrane. It seemed probable, 

 however, that the procedure of Pfeffer failed, in some degree, to accom- 

 plish its purpose, i.e., that the embedding was emperfect; and the ques- 

 tion arose whether it might not be possible to devise some method by 

 means of which these "plugs," of which the membrane should exclu- 

 sively consist, could be more firmly driven into and more securely fixed 

 in their places than is the case when the diffusion method of Pfeffer is 

 employed. It was in this connection that the electrolytic process for 

 the deposition of the membranes occurred to the writer. 



It should be stated, however, that the first suggestion which led up to 

 the solution of the problem came accidentally. While the subject of 

 measuring osmotic pressure was still uppermost in the mind of the 

 writer, he was engaged in an attempt to procure pure aqueous solutions 

 of permanganic acid by the electrolysis of potassium permanganate. 



