54 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



(1) upon the magnitude and rate of the rise in temperature, and (2) upon 

 the time at which the fall in temperature sets in and the rate of the 

 fall. The meniscus then descends to a point below its original level. 

 The distance between the two positions depends (1) upon the magni- 

 tude of the upward displacement and (2) the rate at which the bath 

 recovers its original temperature. Finally, the meniscus ascends to its 

 first place. 



The third and fourth situations are not more simple than the first 

 and second, but enough has already been said for the present purpose, 

 which is merely to emphasize the complex nature of thermometer effects. 

 Hence in the remaining cases the movements of the meniscus only will 

 be stated. 



3. The conditions are as follows: The cell contains a solution of 

 known concentration which is exhibiting its true osmotic pressure at 

 a given constant temperature when a fall in temperature occurs. 

 Afterwards the temperature becomes constant at a lower level. The 

 movements of the meniscus which are observed are : (1) A fall (usually 

 quite rapid) to a point below the position which it will finally take, and 



(2) a rise to some intermediate point at which it becomes stationary. 

 The final position corresponds to the true osmotic pressure, at the given 

 lower temperature, of a 'permanently diluted solution. The difference 

 between the lowest and final positions of the meniscus will depend 

 upon the magnitude of the fall in temperature and upon its rate as 

 compared with that of intake of solvent. 



The movements of the meniscus are sometimes less simple than stated 

 above, since at times one observes an extra excursion of the meniscus, 

 i. e., it falls to its lowest level and then rises to a point above the position 

 which it finally takes. 



4. If, after the fall, the temperature rises and becomes constant 

 again at the original level, which is the most frequent case, the move- 

 ments observed are as follows : The meniscus falls to its lowest position, 

 then rises to one higher than it had originally, and finally sinks to the 

 place from which it started. Here again an extra excursion of the 

 meniscus is sometimes observed, namely, a second one to a point below 

 its final position. When the meniscus has finally recovered the position 

 from which it first started, we have again, of course, the true osmotic 

 pressure of the original solution at the original temperature. 



The "extra" excursions of the meniscus mentioned under 3 and 4, 

 as well as certain other anomalies not mentioned, are probably due to 

 temporary inequalities of concentration in the solution — to the fact, 

 namely, that when solvent is expelled or taken in, the solution in 

 immediate contact with the membrane is, for the time being, concen- 

 trated or diluted to a greater extent than the main body of the solution. 

 The final adjustment of the meniscus can not, of course, be reached 

 until the whole solution has become homogeneous through diffusion 



