THE MEMBRANES. 91 



5. DETERIORATION OF THE MEMBRANE. 



If the ferrocyanide of zinc (the membrane of Tamman) is deposited 

 in a cell in the usual manner and is tested soon thereafter with a 

 solution of sugar, considerable pressure is developed on the first trial, 

 but by no means the full osmotic pressure of the solution. Moreover, 

 the pressure does not at any time become constant, but, having reached 

 its highest development, it falls slowly and continuously until it is in 

 equilibrium with the pressure of the air. If the cell is now emptied 

 and soaked in water, and the membrane is reinforced in the usual 

 manner, and the cell is again set up with a solution of sugar, some 

 pressure is developed, but always less than on the first trial. On each 

 succeeding trial, a still smaller pressure is obtained, until at last the 

 cell develops no pressure whatever. After the first trial, the solutions 

 which are removed from the cell have a milky appearance, due to 

 suspended ferrocyanide of zinc; and on close examination the com- 

 pound is found to have lost its original structureless (colloidal) con- 

 dition and to have become granular, though not distinctly crystalline. 

 We have here a clear case of degeneration which appears to consist in 

 a change in the membrane material from a gelatinous or colloidal 

 condition to a granular state. From the fact that after a time no 

 pressure can be obtained, however much the membrane may be soaked 

 in water or reinforced by the deposition of additional material, it is 

 inferred that during the later experiments the newly formed ferro- 

 cyanide is transformed as fast as it is deposited. A similar, but less 

 striking, degeneration has been noticed on the part of the manganese 

 ferrocyanide membrane. Neither the zinc nor the manganese salt has 

 been found suitable for the measurement of osmotic pressure. 



The ferrocyanide of copper membrane, when carefully treated in 

 the prescribed manner, appears to suffer no such deterioration as long 

 as it is used to measure the pressure of non-electrolytes only, and at 

 moderate temperatures. It is still uncertain whether the disastrous 

 effect of rapid cooling, after using the membrane at high temperature, 

 is due to a similar transformation, or not. The membrane becomes 

 less active with age, but not ineffective. In fact, old membranes are 

 preferred for the measurement of high pressures, because of their great 

 strength and reliability; and old cells are discarded only when the 

 passage of solvent through their membranes becomes intolerably slow, 

 and never because they will not measure correctly, if given time enough. 

 Some cells have been in use more than four years. The decreased 

 activity of old membranes may be due, in part at least, to the thickening 

 effect of the frequent reinforcement with new material to which they 

 are subjected. 



The conduct of the ferrocyanides of nickel and cobalt resembles that 

 of the ferrocyanide of copper. Both of them give membranes which 

 do not appear to degenerate under the influence of non-electrolytes. 



