THE CALORIMETER. 53 



The large majority of the methods for determining the hydrothermal 

 equivalent of materials are at once eliminated when the nature of the calo- 

 rimeter here used is taker, into consideration. Obviously, in warming up 

 the chamber there are two sources of heat : first, the heat inside of the cham- 

 ber; second, the heat in the outer walls. As has been previously described, 

 the zinc wall is arbitrarily heated so that its temperature fluctuations will 

 follow exactly those of the inner wall, hence it is impossible to compute from 

 the weight of the metal the hydrothermal equivalent. By means of the 

 electrical check experiments, however, a method for determining the hydro- 

 thermal equivalent is at hand. The general scheme is as follows. 



During an electrical check experiment, when thermal equilibrium has 

 been thoroughly established and the heat brought away by the water-current 

 exactly counterbalances the heat generated in the resistance-coil inside the 

 chamber, the temperature of the calorimeter is allowed to rise slowly by 

 raising the temperature of the ingoing water and thus bringing away less 

 heat. At the same time the utmost pains are taken to maintain the adia- 

 batic condition of the metal walls. Since the temperature is rising during 

 this period, it is necessary to warm the air in the outer spaces by the 

 electric current. By this method it is possible to raise the temperature of 

 the calorimeter 1 degree or more in 2 hours and establish thermal equi- 

 librium at the higher level. The experiment is then continued for 2 hours 

 at this level, and the next 2 hours the temperature is gradually allowed to 

 fall by lowering the temperature of the ingoing water so that more heat is 

 brought away than is generated, care being taken likewise to keep the walls 

 adiabatic. Under these conditions the heat brought away by the water- 

 current during the period of rising temperature is considerably less than 

 that actually developed by the electric current and the difference repre- 

 sents the amount of heat absorbed by the calorimeter in the period of the 

 temperature rise. Conversely, during the period when the temperature is 

 falling, there is a considerable increase in the amount of heat brought 

 away by the water-current over that generated in the resistance-coil and the 

 difference represents exactly the amount of heat given up by the calorim- 

 eter during the fall in temperature. It is thus possible to measure the 

 capacity of the calorimeter for absorbing heat during a rise in temperature 

 and the amount of heat lost by it during cooling. A number of such 

 experiments have been made with both calorimeters and it has been found 

 that the hydrothermal equivalent of the bed calorimeter is not far from 21 

 kilograms. For the chair calorimeter a somewhat lower figure has been 

 found, i. e., 19.5 kilograms. 



