THE MEMBRANES. 87 



gens," as they are called by Pfeffer, is undoubtedly of service while 

 the development of the membranes under pressure is in progress; but 

 it is still a question whether they are required after the membranes 

 have once been perfected. 



After finishing a measurement of pressure, the cell is emptied, is 

 thoroughly washed, and is then allowed to soak three days or more in 

 water which is 0.001 normal with respect to thymol. The water is 

 renewed at least twice each day. The cell is then ready to be prepared 

 for another measurement of pressure. The preparation consists in sub- 

 jecting it to one or more repetitions of the membrane-forming process, 

 until the high resistance of the membrane indicates that its condition is 

 again satisfactory. 



3. THE SOAKING OF THE CELL. 



It has previously been stated that before every repetition of the 

 membrane-forming or repairing process, the cell (whether it is being 

 prepared for use or is in use) is soaked in distilled water for a consider- 

 able period. This treatment is of the greatest importance — in fact, 

 it can not be dispensed with. Moreover, it may be stated, as a general 

 proposition, that the longer the soaking is continued the better will 

 the condition of the membrane be found to be. In accord with this 

 statement is the fact that those membranes which have soaked through 

 the three summer months without interruption are always found to 

 be in excellent condition for the resumption of work in the autumn. 

 The statement does not, of course, apply to cells which have suffered 

 from infection, or from the effects of use with electrolytes, or from use 

 at high temperatures followed by too rapid cooling. The observed effect 

 of too little soaking is always an inability on the part of the membrane 

 to maintain pressure. 



The beneficial effect of water on the membranes is not fully under- 

 stood, but it is believed to be due to the extraction of alkaline metals 

 from the membrane material, and therefore to the effect which such 

 extraction may be supposed to have in the preservation or improve- 

 ment of the colloidal condition of the membranes. It is the purpose 

 of the writer, while engaged upon this investigation, to confine himself, 

 as much as possible, to the discussion of established facts, but he 

 ventures to suggest in the present connection that all the phenomena 

 which have come under his observation are in accord with the idea 

 that true semipermeability is an attribute of colloids only, and that 

 the passage of water through an osmotic membrane is a phenomenon 

 of the hydration of a colloid upon one side and its partial dehydration 

 on the other. This statement explains nothing, but it enables one 

 to account, in a plausible manner, for the highly beneficial effect of 

 pure water upon the semipermeability of membranes, and also for the 

 deleterious effect of electrolytes. 



