52 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



eter increase. The total pressure is now the sum of the osmotic pres- 

 sure of the solution and a considerable mechanical pressure due to the 

 expansion of the liquid contents of the cell. In consequence of this 

 oi>e?'-pressure — the difference between the total and the osmotic pres- 

 sures — the gas in the manometer is compressed to a smaller volume, and 

 the mercury meniscus is seen to rise and finally to attain to a maximum 

 height. Simultaneously with the expansion of the liquids in the cell, 

 there is, in consequence of the over-pressure, a very slow outward dis- 

 charge of the solvent through the membrane, the effect of which is two- 

 fold. First, there is a reduction in the volume of the solution which 

 reduces the mechanical pressure; and, second, an increase in osmotic 

 pressure due to the increasing concentration of the solution. The two 

 effects are of a mutually compensatory character, but they are not equal 

 in their opposite influences upon the magnitude of the pressure in the 

 cell. Hence the meniscus does not remain at the highest point reached 

 by it, but sinks again and becomes stationary at a lower level only when 

 the mechanical pressure has wholly disappeared and the only pressure 

 in the cell is the osmotic pressure of a solution more concentrated than 

 the original one. To recapitulate, the meniscus, in the case under con- 

 sideration, takes three positions in the manometer, which may be called, 

 in the order of their relative heights, the lowest, the intermediate, and 

 the highest. The first and the second of these correspond to the true 

 osmotic pressures of two solutions of different concentration, while the 

 third is temporary and corresponds to the sum of an unknown mechan- 

 ical pressure and an osmotic pressure of which it can only be said that it 

 is higher than the osmotic pressure of the more dilute and lower than 

 that of the more concentrated solution. 



It will be seen that the maximum height to which the meniscus will 

 temporarily attain depends upon both the magnitude and the rate of 

 the rise in temperature, while its final position is determined solely by 

 the former. In other words, a rapid rise in temperature always pro- 

 duces a larger thermometer effect than a slow one. It will be seen also 

 that the magnitude of the thermometer effect in question, when trans- 

 lated into pressure, depends in large measure upon the volume of the 

 nitrogen in the manometer. 



2. If, after a rise, the temperature, instead of becoming constant, 

 again sinks to its original level, a more complicated series of changes in 

 cell pressure is observed. The cause of the increased complexity of the 

 situation is the falling temperature which may begin to operate before 

 or after the meniscus has reached its greatest height. If it begins 

 before, the meniscus will evidently not rise so high as it otherwise 

 would. For present purposes, let it be supposed that the fall in tem- 

 perature sets in immediately after the meniscus has reached the highest 

 point in its ascent, i. e., when the greatest pressure has been developed 

 in the cell. Up to this point, then, the conditions are identical with 



