154 A RESPIRATION CALORIMETER. 



Theoretically, all material should enter or leave the calorimeter cham- 

 ber at the inside temperature, but in practice it has been found impos- 

 sible to do this ; hence a correction is necessary. 



From the weights of all materials entering the chamber and their 

 specific heats, their hydrothermal equivalent can be readily calculated, 

 which, multiplied by the difference in temperature, gives the amount 

 of heat added to or lost from the chamber. These corrections are made 

 for each experimental period, the data being determined directly from 

 a record sheet posted near the food aperture. 



ADIABATIC COOLING OF GASES. 



With fluctuations in barometric pressure, the air inside the calorim- 

 eter expands or contracts, and consequently liberates or absorbs heat 

 according to the well-known laws of adiabatic cooling. In considera- 

 tion of the large volume of air in the calorimeter, the probable effect 

 of fluctuations in barometric pressure on the amount of heat liberated 

 during a given period has to be considered. 



From data furnished by the chief of the Weather Bureau, 1 it has 

 been computed that a maximum fall of 10 mm. in the barometer is 

 accompanied by a cooling of 1.1, which is equivalent to 1.624 large 

 calories, or 0.1624 calorie per millimeter. This amount of heat is 

 absorbed (rendered latent) as the barometer falls, and liberated as the 

 barometer rises. 



Save in very exceptional fluctuations in the barometer, this correc- 

 tion does not have to be taken into consideration, and thus far has not 

 been necessary. It is possible, however, that in rest or fasting experi- 

 ments, in which the amounts of heat liberated are small, this correction 

 may amount to a percentage of the whole so large that it should be 

 allowed for. 



CORRECTION FOR HEAT ABSORBED BY BED AND BEDDING. 



When the subject retires (at 1 1 p. m. ),the heat radiated from the body 

 is absorbed by the bed and bedclothes till the temperature of the por- 

 tions nearest his body are warmed from chamber temperature (20) to 

 approximately that of the body (35). As a result, the heat measured 

 from ii p. m. to i a. m. is too low. On the other hand, when the sub- 

 ject leaves his bed (at 7 a. m.), the bed and bedding again cool down to 

 the temperature of the chamber, and the heat measured from 7 a. m. to 

 9 a. m. is too high. In determining the heat output by periods, correc- 

 tion should be therefore made for heat stored in this way. The data 

 available for estimating the exact amount of this heat are by no means 



1 U. S. Weather Bureau, Report (1899), 11, p. 492. 



