86 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



A cell which has been used at a high temperature, and has then been 

 allowed to cool rapidly and to stand at a considerably lower temper- 

 ature, is generally ruined for use at any temperature whatsoever. An 

 unfortunate experience of the writer and one of his associates will illus- 

 trate the point. Starting with about 25 cells at 25°, they had labor- 

 iously trained them up by short temperature-intervals, until they were 

 measuring with perfect success at 70° and 80°. When the summer 

 vacation arrived, the cells were put in soak in thymol water, as usual at 

 such times, and allowed to stand through the summer months. On 

 resuming the work in the autumn, it was found impossible to restore the 

 cells to a usable condition at high temperatures, and only a few of them 

 were afterwards useful at any temperature. It is not known what hap- 

 pened to the membranes in consequence of the large and rather rapid 

 temperature transition. It is possible that they might have been saved 

 by reversing the process by which they were built up for use at high 

 temperatures, i. e., by dropping the temperature gradually and "season- 

 ing" the cells into a usable condition at short temperature-intervals. 

 Membranes which have been prepared and used at any ordinary tem- 

 perature withstand the fluctuations of summer temperature without 

 deterioration. 



2. TREATMENT OF THE CELL WHILE IN USE. 



The foregoing statements have especial reference to the preparation 

 of the cell for the measurement of pressure. The treatment of the cell 

 while in use has not been stated in sufficient detail. Having built up 

 the membrane, in the manner described, until its resistance can be 

 forced no higher, and having afterwards " seasoned" it under pressure 

 until the solutions are proved to suffer no dilution while in the cell, 

 the formal measurements of osmotic pressure are begun. The first 

 statement to be made in this connection is the general one that, from 

 the time the deposition of the membrane is commenced until the work 

 of measuring at a given temperature is finished, the cell is maintained, 

 as nearly as possible, at te?nperature. This makes it necessary also 

 to maintain at temperature all the solutions which are used with it. 



Following the custom of Pfeffer, there is added to the solution whose 

 pressure is to be measured a small amount of potassium ferrocyanide. 

 The exact quantity is 83.9 milligrams to each 100 grams of water, 

 which gives a 0.01 weight-ion-normal solution, if the dissociation of 

 the salt is complete. This solution is one-tenth as strong, with respect 

 to the ferrocyanide, as that formerly used in depositing the membranes, 

 and is of the same strength as that which is employed in developing 

 them under pressure. An osmotically equivalent quantity of copper 

 sulphate (123.9 milligrams per 100 grams of water) is added to the 

 water in which the cell stands during an experiment. The solutions 

 without and within the cell are also made 0.001 weight-normal with 

 thymol to guard against infection. The presence of the "membrano- 



