88 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



The suspicion that much of the difficulty experienced in securing 

 good membranes and in maintaining them in effective condition there- 

 after is due to the mischievous influence of potassium upon the colloidal 

 state of the membrane material led to several modifications of the 

 method of building up membranes. The first of these was a consid- 

 erable dilution of the solutions which are used in forming them, a 

 dilution from 0.1 to 0.01 weight-ion-normal. Somewhat later the con- 

 centration of the "membranogens," which are used within and without 

 the cells while measuring pressure, was reduced from 0.01 to 0.001 

 normal. The effects were seemingly good, in that higher resistances 

 could be obtained at any single "running" than before and the cells 

 appeared to require less soaking between measurements. Still later a 

 0.01 weight-ion-normal solution of ferrocyanic acid was substituted for 

 the potassium salt in all operations connected with the deposition and 

 reinforcement of membranes. The results of the last substitution are 

 very promising, and the acid seems likely wholly to displace the salt. 



It has been found that, while potassium salts are in a marked degree 

 injurious to the membranes and may easily ruin them, the salts of 

 lithium are comparatively harmless. This discovery has been utilized 

 to some extent and with advantage in the deposition of membranes by 

 substituting the ferrocyanide of lithium for that of potassium. 



4. ACTIVITY OF THE MEMBRANE. 



In discussing the so-called "thermometer effects," which are due to 

 fluctuations of temperature, the fact was emphasized that the passage 

 of solvent through the membrane is by no means instantaneous, and 

 that it may be exceedingly slow. We have, therefore, a "barometer 

 effect" due to fluctuations of atmospheric pressure. Barometer effects 

 are, however, less troublesome than thermometer effects, because, as 

 a source of error in the measurement of osmotic pressure, their mag- 

 nitude is limited to the comparatively small variations in atmospheric 

 pressure. Moreover, the errors due to them can be eliminated by 

 correcting the mean of all the daily observations of osmotic pressure 

 by the mean barometric pressure for the whole period within which 

 an experiment is in progress. 



The activity of a membrane, i. e., the rate at which the solvent will 

 pass through it, depends, of course, in the first place, upon its area. 

 Since, however, the membrane is made up of a multitude of little 

 "plugs," which fill the mouths of the pores opening upon the interior 

 of the cell, the area of the membrane is equal to the aggregate area 

 of these pores at their mouths. In other words, the size of the mem- 

 brane depends, not on the area of the interior of the cell, but upon the 

 number and the size of the pores in the cell wall. It is this fact which 

 makes all quantitative comparisons of the membranes of different cells 

 impossible. We can never know their relative areas. It is only known 

 that, other things being probably equal, water passes more slowly 



