136 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



accomplish the main purposes immediately in view — namely, the com- 

 plete suppression of thermometer effects and dilution — the whole method 

 must be extensively improved. 



The revision which followed, previous to beginning Series V, was a 

 radical one, which affected nearly every detail of the procedure. The 

 more important of the measures taken at that time for the elimination 

 of dilution have already been mentioned. The method of wrapping 

 the exposed part of the stopper was changed, with the result that in 

 Series V and in the succeeding series there were no upward displace- 

 ments of the manometers. In other words, the capacity of the cells 

 no longer increased under pressure, and one of the three sources of 

 dilution — though probably, in the beginning, the smallest — had at last 

 been eradicated. The practice of " dipping" the cells, before closing 

 and opening them, was followed systematically, and the method of 

 piercing and "slitting" the stopper, before removing the manometer, 

 was greatly improved. It was at this time also that nitrogen was 

 substituted for air in the manometers, and that more attention began 

 to be given to the errors in measurement which are due to the irregu- 

 larities of capillary depression in narrow tubes. 



The improvements in the devices for bath regulation had in view the 

 bringing of the whole system of temperature control into harmony with 

 the general scheme which has been formulated in a previous chapter in 

 the following words: 



"If all the water or air in a bath is made to pass rapidly (1) over a con- 

 tinuously cooled surface which is capable of reducing the temperature slightly 

 below that which it is desired to maintain, then (2) over a heated surface which 

 is more effective than the cooled one, but which is under the control of a 

 thermostat, and (3) again over the cooled surface, etc., it should be practicable 

 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 essential features of this scheme — the cooling and heating sur- 

 faces and the circulation of the air or water between them — are not 

 novel. They are exemplified in part or fully, and more or less perfectly, 

 in nearly all baths. But perfect success in temperature regulation 

 depends upon the simultaneous and harmonious cooperation of all 

 three. In principle, it makes no difference whether the heating or 

 cooling agent is subjected to exact regulation by a thermostat. 



In Series I, the maintenance of temperature was by insulation. 

 There was no thermostat in the system — unless the insulation can be 

 considered in that light — and the walls of the bath became therefore 

 an uncontrolled heating or cooling surface according to the temperature 

 of the surrounding air. 



In Series II, the heating surface was provided by the electric stoves, 

 which were regulated by a thermostat. The other essential — the cooling 

 surface — was furnished by the walls of the bath ; but these became an 



