THE MEMBRANES. 



89 



through a membrane in a hard-burned cell, in which the pores are 

 presumably small, than it does through the membrane of a soft-burned 

 one, in which the pores are presumably large. 



The rate at which water will pass through a given membrane de- 

 creases with age and use. In cells with new membranes, the osmotic 

 pressures of solutions often attain a maximum within six hours, pro- 

 vided the temperatures, and therefore the volumes of the solutions, 

 remain constant; while the same cells, two years later, may require 

 from five to ten days, or even longer, for the establishment of equi- 

 librium pressures. In Table 4 the records of the determinations of 

 osmotic pressure illustrate the difference, as regards the time required for 

 the development of equilibrium, between an excellent new membrane 

 and an old one, which, though slow, is otherwise in good condition for 

 the measurement of osmotic pressure : 



Table 4. — Observed osmotic pressures. 



I. New membrane. 



0.9 weight-normal solution of 



cane sugar. Temperature 25°. 



First day 24.127 



Second day 24.148 



Third day 24.125 



Fourth day 24.102 



Fifth day 24.125 



Mean 24.126 



II. Old membrane. 



0.6 weight-normal solution of 



cane sugar. Temperature 25°. 



Sixth day 15.654 



Seventh day 15.612 



Eighth day 15.628 



Ninth day 15.629 



Mean 15.627 



The maximum pressure was reached in less than six hours in the 

 case of the new membrane cited above, while six full days were required 

 in that of the old one. In another instance, the maximum pressure 

 was reached on the tenth day and it remained constant for 12 days, 

 when the cell was opened. The measurement was an excellent one, 

 and the only defect of the cell was the excessive slowness with which 

 the solvent passed through the membrane into the solution. It should 

 also be noted in this place that certain cells whose membranes had 

 suffered some deterioration through contact with electrolytes have been 

 known to require more than 20 days for the establishment of final 

 pressures. The measurements were, nevertheless, entirely satisfactory. 



In cells with old membranes, and in those whose membranes have 

 become slow through contact with electrolytes, "thermometer" and 

 "barometer" effects are necessarily large; hence when small pressures, 

 i. e., those of dilute solutions, are to be measured, cells with young 

 and especially active membranes are selected. For such purposes, 

 soft-burned cells have the obvious advantage that in them the areas 

 of the membranes are relatively large. If the pressures to be measured 

 are minute, they may be entirely masked by the thermometer and 

 barometer effects. To illustrate this point, it is recalled that certain 



