152 A RESPIRATION CALORIMETER. 



experiments, and leaves the chamber, after having passed through the 

 absorbing pipes, at 12, the result will be in terms of C (z -i 2) or in terms 

 of the mean calorie from 2 to 12. From the table above referred to 

 it is found that the specific heat at 2 is 1.0076 and at 12, 1.0020. 

 The average of these two is 1.0047. This variation is approximately 

 0.5 per cent. Since the accuracy of the calorimetric measurements is 

 considerably within i per cent, it is evident that the correction above 

 suggested must be applied. 



In making the correction, the quantity of heat measured in terms 

 of C t is multiplied by the specific heat of water at C t referred to that 

 at C M as a standard. 



CORRECTIONS TO MEASUREMENTS OF HEAT. 



As explained above, to obtain the true final measurement of heat, 

 allowance must be made for certain quantities of heat introduced or 

 removed in various ways. The different corrections to be made are dis- 

 cussed in the following sections. 



THE HYDROTHERMAL EQUIVALENT OF THE CALORIMRTER. 



With the heat- regulating devices previously described, it is in gen- 

 eral not at all difficult to control the temperature of the calorimeter 

 within very narrow limits ; but there are times when the calorimeter 

 system, as a whole, may have a different temperature at the end of a 

 period than at the beginning, and there may be accordingly either a 

 storage or a loss of heat in the system. Obviously, in accurate experi- 

 menting, especially in short periods, it is necessary to know the actual 

 amount of heat thus stored or lost. This involves a knowledge of the 

 hydrothermal equivalent of the calorimeter, since the mass of material 

 thus raised or lowered in temperature must be known and expressed in 

 its equivalent weight of water. 



With a calorimeter of this type of construction it is not an easy matte r 

 to determine the hydrothermal equivalent with great accuracy. The 

 inner copper wall is heated by the heat radiating from the subject. 

 The outer zinc wall is heated by the electrical current in the air-space 

 surrounding it. If the chamber undergoes a certain rise in tempera- 

 ture, it is difficult to state exactly what proportion of the heat given off 

 by the subject is utilized in raising the temperature of the copper wall 

 and what proportion is utilized in raising the temperature of the zinc 

 wall, for while there is obviously a distinct period during which the 

 copper wall is warmer than the zinc wall, it is by no means absolutely 

 certain that when the temperature is rising all the heat from the man's 

 body escapes to the zinc wall before the electrical heating circuit begins 



