56 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



of 0.02° in bath temperature, if they follow one another with regularity, 

 are tolerable, because the thermometer effects due to rise in tempera- 

 ture are then partially neutralized by those due to falling temperature. 

 Change in concentration without leakage. — It has been seen that a 

 solution in a cell may become permanently concentrated if the tem- 

 perature rises and becomes constant at a higher level; also that it 

 may be permanently diluted if the temperature falls and becomes 

 constant at a lower level. There are also two cases in which the con- 

 centration of the solution may be altered without any change in the 

 temperature of the bath. Alterations of this kind occur when the 

 cells are filled with solutions whose temperature differs from that of 

 the bath. In such cases, concentration or dilution of the solution 

 ensues, according as the temperature of the solutions is lower or higher 

 than that of the bath. There is, however, no essential difference 

 between the two modes of effecting concentration on the one hand 

 and dilution on the other, since both depend on changes in volume due 

 to changes in the temperature of the solutions. 



Except for the maintenance of zero temperature, all the devices for 

 regulation conform to one principle, which may be stated as follows : 



// all the water or air in a bath is made to pass rapidly (1) 

 over a continuously cooled surface which is capable of reducing 

 the temperature slightly below that vjhich it is desired to main- 

 tain, then {2) over a heated surface which is more efficient than 

 the cooled one but which is under the control of a thermostat, 

 and (3) again over the cooled surface, etc., it should be practi- 

 cable to maintain in the bath any temperature for which the 

 thermostat is set, and the constancy of the temperature should 

 depend only on the sensitiveness of the thermostat and the rate 

 of flow of the water or air. The principle is a general one and 

 provides for the maintenance of any temperature between zero 

 and the boiling-point of water. Moreover, any desired tempera- 

 ture can be maintained without regard to the temperature of the 

 surrounding atmosphere, since the air about the bath must always 

 aid in the work either of the cooling or the heating surface. 



The "cooling" surface is usually furnished by a series of brass pipes 

 through which water — under a constant pressure — is circulated. If the 

 temperature to be maintained is a moderate one, i. e., not far from that 

 of the atmosphere but above that of the hydrant water, the latter is 

 passed directly through the circulating system, the rate of flow being 

 so regulated as to maintain, without the cooperation of the heating 

 surface, a temperature which is slightly too low. This margin between 

 the temperature which the cooling surface, acting alone, will maintain 

 and that which it is desired to keep should, for economical reasons, be 



