210 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



been observed to reach a maximum. It has been stated in general 

 terms that "the longer the soaking is continued, the greater is found to 

 be the improvement of the membranes"; also, that those membranes 

 which have remained submerged in pure water through the three 

 summer months are usually in excellent condition for the resumption 

 of work in the fall. But the most notable demonstration of the value 

 of water as a restorative was observed in connection with certain cells 

 which, after having been used for some time at high temperatures, 

 were allowed to cool quickly down to ordinary temperatures. The 

 subsequent history of these cells has been partly told in earlier chapters, 

 but not all of it. More than three months were spent in trying to 

 restore them to usable state, i. e., to reproduce the semi-permeable 

 condition of the membranes, but without success. They have since 

 remained in water continuously up to the present time. Occasionally, 

 they have been tested as to the state of the membranes by setting them 

 up with solutions of cane sugar or glucose. At the end of twelve 

 months, one of the cells began, to our surprise, to develop and to main- 

 tain the full osmotic pressures of the solutions. During the following 

 five months, two others were found to be in suitable condition for the 

 measurement of pressure; and during the eighteenth month, several 

 more were brought into use. The most obvious explanation of the 

 effect of pure water on the semi-permeable state of the membranes is 

 that it preserves and improves their colloidal condition. 



5. The auto-degeneration of the membranes. — By "auto-degeneration " 

 is meant the loss of semi-permeability which is observed in such mem- 

 branes as the ferrocyanide of zinc and the ferrocyanide of manganese. 

 These are moderately active in the beginning, but soon become less so, 

 and within a short time they lose every vestige of the semi-permeable 

 character. To the term " auto-degeneration ," there should, perhaps, 

 be added that of induced-degeneration to cover a phenomenon which is 

 also observed in the case of zinc ferrocyanide, the fact, namely, that when 

 the membrane has once lost its semipermeability, all later deposits of 

 membrane material immediately lose their osmotic activity. In such 

 cases there is a change in the condition of the material, which appears to 

 consist in a passage from the colloidal to a granular or crystalline state. 

 In the presence of water, and in the absence of electrolytes, membranes 

 consisting of the ferrocyanide of copper, nickel, or cobalt do not appear 

 to be subject to what is here styled "auto-degeneration." 



Persuaded as we were (by the large amount of seemingly pertinent 

 evidence, which had been gathered while we were measuring the pressures 

 of non-electrolytes) that the semipermeability of the membranes de- 

 pends on their colloidal condition, and that the possibility of measuring 

 osmotic pressure depends upon the maintenance of that state — the 

 attempt to measure the pressures of electrolytes was begun with great 

 misgivings. It was, nevertheless, determined to test the copper ferro- 

 cyanide membrane with various salts; and in case of failure to institute 



