THE ENERGY OF THE METABOLISM 181 



companiment of their metabolism. The determination of 

 such data requires the use of calorimeters. - These are of 

 various forms, but have the same underlying principle. In 

 case a food sample is to be burned it is enclosed, in a 

 small chamber, preferably in an atmosphere of oxygen, and 

 its combustion is initiated by means of an electric spark. 

 The heat is imparted to a large mass of water surrounding 

 the chamber and can then be estimated by the elevation of 

 temperature observed in this water. This is an elementary 

 and somewhat sweeping statement; numerous corrections 

 would be necessary in practice. 



One of the earliest attempts to gage the heat production 

 of an animal consisted in confining it within a chamber 

 having double walls and ice between. The amount of ice 

 melted could thus be made the basis for the estimate de- 

 sired. The employment of a water calorimeter evidently 

 does away with the unnatural chilling which must have 

 been entailed in this primitive trial. The large and costly 

 calorimeters in modern laboratories for the study of human 

 nutrition are of the water type. No adequate idea of the 

 difficulty involved in such work is likely to be grasped by 

 reading this condensed presentation. Ventilation must be 

 maintained and allowance made for the warming of the 

 circulating air. The evaporation of water must be ac- 

 curately measured, for if this were ignored a great deal of 

 heat produced in the metabolism would escape measure- 

 ment. (In the evaporation of 1 kilogram of water heat to 

 the amount of 536.5 Cal. will be made latent.) Changes 

 in the body temperature cannot be overlooked. If, for 

 example, a human subject equivalent in heat capacity to 

 50 kilograms of water should experience a rise of tempera- 

 ture amounting to 0.5 C. during an experiment, 25 Cal. 

 would have been retained in his body. If, instead, there 

 were a fall of 0.5 degrees there would have been a discharge 

 of 25 Cal., and the observed heat output would have been 

 greater than that due to the current metabolism. 



Now the question may be asked whether we can be sure 

 that the observed heat production (corrected for body 



