328 



BABCOCK. 



amount of current is passed through the heater for a certain time, 

 and the change of temperature of the calorimeter observed. The 

 mean heat capacity of the entire apparatus over this temperature 

 interval is obtained by dividing the energy input by the temperature 

 change produced by this energy. Since the rate of change of heat 

 capacity does not vary appreciably during a heating of one degree, 

 the value obtained is taken as the heat capacity at the mean tempera- 

 ture of the experiment. A series of such measurements are made at 



various temperatures, and the heat capacit;!>-, denoted by 



■Q' 



IS 



plotted as a function of the temperature. 



To correct for the heat capacity of the system exclusive of the 

 ammonia, the liquid container is emptied, and the series of measure- 

 ments repeated. The heat capacity of the empty apparatus,* denoted 



/FiTJ \ 

 by ( - — - ) , is plotted as a function of the temperature, and this curve 



constitutes a calibration of the calorimeter. The trend of these — - 



8d 



FiGXJKE 1. 



curves is shown in Figure 1. At any temperature dx, the heat cap- 

 acity of the ammonia is measured by the difference of the ordinates 



8Q 



to the two curves, and this difference is denoted by 



8d 



To obtain the specific heat capacity of the ammonia, it is necessary 



80 

 to divide the values of — , at each temperature by the mass, M, of 



89 



ammonia in the container. The volume of this container is sensibly 



4 The correction for the heat capacity of the air in the container is negligible. 



