218 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



gish. Hitherto, diminished activity on the part of membranes has 

 usually been the result of age and frequent use. But the membranes 

 which were employed for the measurement of the osmotic pressure of 

 the lithium salt were new ones, and they had not been used for any 

 other purpose. With either cane-sugar or glucose solutions, they should 

 have given equilibrium pressures within one or two days. With solu- 

 tions of lithium chloride, the shortest time required for that purpose 

 was 9 days, while the average time consumed in developing the final 

 pressures was 17 days. The membranes were not wholly ruined by 

 their contact with the electrolyte, as others had been by potassium 

 chloride; for they were afterwards successfully employed for the meas- 

 urement of the osmotic pressure of mannite solutions. But the state 

 of inertness which they had acquired in the presence of the lithium salt 

 persisted without diminution throughout their later history. Event- 

 ually, the cells were withdrawn from use, because of their slowness, and 

 consigned to a solution of thymol, in order to ascertain whether the 

 membranes might not recover their normal activity under the influ- 

 ence of water. This is the course which is now taken with all slow 

 cells whenever their long-continued monopolization of bath space and 

 manometers becomes intolerable. Many membranes do recover a fair 

 degree of activity under such treatment, though the time required for 

 restoration is usually very long — sometimes more than two years. 



Particular attention is called to experiment 2 with the 0.4 weight- 

 normal solution. This was an endurance test of the membrane of an 

 unusually thorough character. The cell (F„), at the time of setting 

 it up, had a resistance of 1,100,000 ohms, and it remained in the bath 

 145 days. Starting with an initial pressure of 15 atmospheres, it 

 reached an approximate equilibrium in 10 days. The osmotic pressure 

 which the cell sustained during the following 125 days is given in 5 

 columns, each of 25 daily records. The mean osmotic pressure for 

 the first period was 18.827; for the second, 18.894; for the third, 18.799; 

 for the fourth, 18.636; and for the fifth, 18.405. It is believed that a 

 mean of the records for the first 100 days fairly represents the osmotic 

 pressure of the solution. But during the fifth period, i. e., from the 

 101st to the 125th day of the record, there was a decline in pressure from 

 18.609 to 18.140 atmospheres, which can only signify that the membrane 

 had at last begun to weaken. The cell was allowed to remain 10 days 

 longer in the bath, but it gave no evidence of recovering any portion of 

 the loss sustained during the fifth period ; in fact, the rate of decline in 

 pressure increased quite perceptibly. The membrane of cell F 6 had 

 evidently suffered severely from its long contact with lithium chloride; 

 for it was found unfit for further measurements of pressure. This does 

 not mean, of course, that it can never be restored to usefulness. 



The very considerable resistance of the membrane to the electrolyte, 

 which was exhibited in the case of the endurance experiment with the 



