THE MEMBRANES. 85 



above the known osmotic pressure of the solution or considerably below 

 it. In the former case the mercury in the manometer falls, in the latter 

 it rises. Eventually, in either case, the meniscus generally comes to 

 rest at a point below the position it should have, showing that the solu- 

 tion has been diluted. The irregular movements of the meniscus, 

 which often precede the assumption of its final position, are all such as 

 can be interpreted as being due to a breaking of the membrane under 

 pressure, and a mending of the rents by the membrane-formers. What- 

 ever may be the result of the first trial, which usually extends over sev- 

 eral days before the pressure becomes constant, the cell is emptied and 

 soaked from 3 days to a week in distilled water, when it is again sub- 

 jected to the membrane-forming process. Afterwards, it is again set 

 up with a solution of cane sugar in the same manner as for the first trial. 

 On the second trial the pressure developed is usually higher than on the 

 first. Sometimes, but not often, the full osmotic pressure of the un- 

 diluted solution is obtained. The further procedure with the cell is 

 simply a repetition of the steps already described. Sooner or later there 

 is developed in this way a cell which gives maximum pressures on every 

 occasion, and in which the solutions suffer no dilution. It is then ready 

 for the measurement of osmotic pressure. 



OBSERVATIONS ON THE MEMBRANE. 

 1. TEMPERATURE OF DEPOSITION. 



As nearly as possible, the membrane is deposited and developed at the 

 temperature at which it is afterwards to be employed for the measure- 

 ment of pressure. The temperature rises somewhat during the deposi- 

 tion, but is kept within limits by the frequent renewals of the ferro- 

 cyanide solution. During the intervals of rest, i. e., while soaking, the 

 cell is also maintained at the temperature at which its membrane was 

 formed and at which it is to be used for the measurement of pressure. 



When a cell has been prepared or used at one temperature and is to 

 be prepared for use at another, the ease with which the change may be 

 accomplished depends very much upon the relation of the different tem- 

 peratures to one another, and whether they are, in general, high or low 

 temperatures. It has been found that when the temperatures in ques- 

 tion are moderate ones, e. g., between 0° and 30°, it is better to deposit 

 the membrane and use the cell at the highest temperature first, and then 

 to work at each of the lower temperatures in the descending order. The 

 reverse order is, however, entirely practicable. If the membranes are 

 to serve for measurements at high temperatures, e. g., 50° to 90°, the 

 training of the cells for their work is quite laborious. It is then neces- 

 sary to deposit and "train" the membranes at some moderate temper- 

 ature, e. g., 30°, and to repeat these operations at short temperature- 

 intervals in the ascending order. 



