90 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



small amounts of the membrane-forming compounds are employed 

 in measuring osmotic pressure for the purpose of mending any rents 

 which may be made in the membranes. The compounds, in the quanti- 

 ties used, are supposed to be osmotically equivalent, and therefore 

 to have no effect upon the observed osmotic pressures ; but this can not 

 be proved, because the difference, if any, is less than the unavoidable 

 thermometer and barometer effects. Similar difficulties are encoun- 

 tered when it is attempted to employ very small membranes. On 

 certain occasions, in the course of the present investigation, it was 

 desirable to employ membranes of very limited area. To obtain these, 

 the cells were glazed over the whole surface, interior and exterior, and 

 afterwards ground off on opposite sides of the cell over as much of the 

 "biscuit" as it was desired to expose for the membrane. Such cells 

 were found, however, to be quite impracticable, because of their slow- 

 ness in responding to fluctuations of bath temperature and atmospheric 

 pressure. Since, other things being equal, the time required for the 

 establishment of equilibrium pressure and the magnitude of the ther- 

 mometer and barometer effects are inversely proportional to the area 

 of the membrane, it is obviously desirable to make the membranes as 

 large as they may be consistently with other requirements. 



"Slow cells" are kept under observation for much longer periods 

 than "quick" ones, because of the minimizing effect of time on the 

 magnitude of thermometer and barometer effects. 



After hundreds of quantitative measurements of osmotic pressure, 

 we are still unable to say how the rate of passage of the solvent through 

 the membrane is affected by the concentration of the solution within 

 the cell. The difficulty in settling a question of this kind is due to 

 the fact that no two membranes are exactly alike in all the elements 

 which determine the rate of transference. It has already been stated 

 that the membranes of no two cells are of equal area; and it is also true 

 that the membrane in any given cell is never exactly the same on two 

 successive occasions. The effect of temperature upon the activitjr of the 

 membranes is also uncertain, though the writer and his associates are 

 under the impression that a rise in temperature increases their activity. 

 But here again quantitative comparisons are impossible. 



If a cell is set up with a solution, under a small initial mechanical 

 pressure, it is noticed that the rise of the mercury in the manometer 

 is very rapid at first, but that the rate of ascent decreases with great 

 regularity and becomes exceedingly slow as the meniscus approaches 

 its final position. This appears to indicate that the rate of passage 

 of the solvent through the membrane depends on the difference between 

 the pressure existing in the cell and the true osmotic pressure of the 

 solution. There are, however, no means of determining how the whole 

 time required for the establishment of equilibrium is related either to 

 the concentration or to the temperature of the solution. 



