164 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



consequences to the cells were apprehended from this treatment; for in 

 all previous work at moderate and low temperatures it had been found 

 that — provided adequate measures were taken to prevent infection — the 

 membranes were greatly improved by the customary summer soaking in 

 water free from electrolytes. We were, therefore, wholly unprepared for 

 the calamity which resulted (apparently) from too rapid cooling of the 

 membranes from 80° to the temperature of the air. On resuming work in 

 the fall, it was found that none of the membranes would sustain the full 

 osmotic pressures of the solutions either at high or moderate tempera- 

 tures. More than three months were spent in applying all the known 

 means for the restoration and improvement of membranes, but to no 

 purpose. None of them could be made to measure pressure at any tem- 

 perature. It was evident that the material of the membranes had under- 

 gone some change in structure which robbed them, at least to a great 

 extent, of their semipermeable character. Moreover, it was to be inferred 

 from the impossibility of building up good new membranes in the presence 

 of the old ones, that probably a similar transformation was quickly induced 

 in all newly deposited membrane material. 



Having found that cells which had formerly been in excellent condition 

 for work at high temperatures could not be restored to a usable condition, 

 they were consigned to a solution of thymol in order to test the effect of 

 prolonged soaking in water. It was then necessary to begin again at 

 the bottom, that is, to make new cells, to build up in them membranes at 

 some moderate temperature, and afterward to perfect these membranes at 

 higher and higher temperature-intervals. The preparation of the clays, 

 and the making, burning, and glazing of the cells require considerable 

 time, but by no means as much as the "training" of the membranes for 

 work at high temperatures. For that purpose it is necessary to deposit 

 the first membranes at a low or moderate temprature, probably not above 

 30°, and then to develop them at that temperature until they are found 

 to measure osmotic pressure satisfactorily. Though measuring perfectly 

 at 30°, they will be found defective at 40°, and must be again developed 

 at the latter temperature, etc. The cells with which the work reported 

 in this chapter is to be resumed at 70° are now (15 months after begin- 

 ning their manufacture) measuring satisfactorily at 50°. 



In the following statement of the results obtained between 0° and 

 80°, the few data which accompany each of the 270 records of observed 

 pressures, namely, the cell used, the resistance of the membrane, and the 

 "initial" pressure, are included because they serve to illustrate many of 

 the points which have been made in previous chapters. After these, are 

 given the mean daily pressures, beginning with the day on which the pres- 

 sure was supposed to have reached a close approximation to equilibrium. 

 Several readings were made each day, and it is the mean of all of these 

 which is to be understood by the term "mean daily pressure." Between 

 0° and 25° it was customary to correct the mean of the total pressures of 



