THE MEMBRANES. 



93 



it up, the osmotic pressure which it registered was 12.522 atmospheres, 

 the barometric pressure being 1.013. On the sixtieth day thereafter, 

 the osmotic pressure was also 12.522 atmospheres and the barometric 

 pressure was 1.016. The lowest osmotic pressure observed during the 

 two months was 12.517; the highest was 12.552. The extreme (appar- 

 ent) fluctuation in osmotic pressure was therefore 0.035 atmosphere, 

 or 0.28 per cent of the mean (12.533 atmospheres) of all observations. 

 The extreme fluctuation in atmospheric pressure during the same period 

 was 0.035 atmosphere. In other words, the extremes of variation were 

 the same for osmotic and barometric pressures. The highest (apparent) 

 osmotic pressures were contemporaneous throughout with the lowest 

 atmospheric pressures, and vice versa. This is due, of course, to the 

 fact that, owing to the slowness with which the solvent passes through 

 the membrane, the pressure within the cell can not immediately adjust 

 itself to changes in atmospheric pressure. 



8. REMOVAL OF THE MEMBRANE. 



When, through age and frequent reinforcement, or through the dete- 

 rioration due to contact with electrolytes, a membrane has become 

 intolerably "slow" or otherwise unserviceable, it is desirable to replace 

 it by a new one. According to Pfeffer,* an old membrane of copper 

 ferrocyanide can be removed and successfully replaced by a new one. 

 For the removal, he recommends soaking the cell in a dilute solution 

 of potassium hydroxide, to which has been added a little Rochelle salt, 

 and afterwards in water, in hydrochloric acid, and again in water. 

 The removal of the membrane by this method is easy, but we have 

 never been able, after such treatment of a cell, to build up in it a 

 thoroughly good new membrane. The process has been modified in 



*"Osmotische Untersuchungen," 12. 



