THE REGULATION OF TEMPERATURE. 53 



those in the preceding case. But there is now a falling instead of a 

 stationary temperature. The elements of the situation are (a) an over 

 or mechanical pressure in the cell due to a previous rise in temperature, 

 and (6) a falling temperature. The consequences of (a) are: 



(1) An increase in osmotic pressure, due to the concentration of the 



solution which follows the expulsion of solvent. 



(2) A decrease in mechanical pressure, due to the smaller volume 



of the solution after expulsion of solvent. 

 The consequences of (b) are: 



(3) A decrease in mechanical pressure, due to the diminishing 



volume of the cell contents. 



(4) A decrease in osmotic pressure, due to lower temperature. 



(5) A decrease in osmotic pressure, due to dilution of the solution 



through intake of solvent. 



(6) An increase of pressure within the cell, due to the increase in 



the volume of the solution through intake of solvent. 

 Of the effects enumerated above, (1) and (6) are positive, i. e., they 

 tend toward the maintenance or increase of pressure in the cell. In the 

 same sense, (2), (3), (4), and (5) are negative. The amount of over or 

 under pressure in the cell at any given moment is, of course, the alge- 

 braic sum of all these effects. By "over" and "under" pressure is 

 meant the difference between the actual pressure in the cell at any time 

 and the true osmotic pressure of the solution at the original temper- 

 ature, i. e., before the rise and subsequent fall of temperature. One 

 would expect, perhaps, that the sum of the "over" and "under" pres- 

 sures would become zero when the bath had recovered its original tem- 

 perature. In other words, that the meniscus would stop in its descent 

 and become stationary, when the pressure in the cell is equal to the true 

 osmotic pressure of the solution at the original and now constant tem- 

 perature. But this is by no means the case. It continues to descend, 

 and, before coming to a rest, may go far below the level which corre- 

 sponds to the osmotic pressure of the original solution at the given 

 temperature. Here again the reason for the apparently anomalous 

 conduct of the cell is to be found in the fact that changes in volume, due 

 to fluctuations of temperature, are accomplished more quickly than the 

 migrations of solvent through the membrane which follow such fluctua- 

 tions. Having reached the lowest point in its descent, the meniscus 

 rises again and finally comes to rest at its original level, i. e., at the level 

 which corresponds to the true osmotic pressure of the original solution 

 at the original temperature. The upward movement of the meniscus 

 in the final return to its first position is the resultant of two opposite 

 effects of the intake of solvent: (1) the increasing volume and (2) the 

 decreasing osmotic pressure of the solution. To recapitulate : If, after 

 a rise, the bath recovers its original temperature, the meniscus first 

 ascends to a point whose elevation above the original position depends 



