54 



PHYSIOLOGY 



CHAP. 



hermetically closed, filled with oxygen under a pressure of 25 

 atmospheres, and then is entirely immersed in the water of the 

 calorimeter A. When uniform temperature has been established, 

 the electric current is passed through the spiral wire, which 

 becomes incandescent and sets the substance on fire ; the com- 

 bustion owing to the high pressure of oxygen is instantaneous and 

 complete, the products of oxidation being carbon dioxide and 

 water. 



If the capacity for heat of the water in the calorimeter has 

 been previously determined by burning in it a definite quantity 

 of a substance with a known heat value, and also the remaining 

 constants of the apparatus, including the weight and heat pro- 

 duced in the spiral wire, the heat of combustion of the substance 

 under examination can readily be calculated from the data 

 obtained. 



The following table shows the results obtained by the three 

 authors already mentioned for the thermogenic value of the most 

 important organic food substances ; the figures relate to 1 grm. 

 of the substance, and the heat of combustion is expressed in 

 large calories (1 cal. = the heat required to raise the temperature 

 of 1 litre of water from to 1 C.). 



1 Without fat and after removal of the extractives by boiling. 



2 Extracted with water, alcohol, ether, and free from ash. 



In the same way as in the calorimeter, but much more slowly, 

 the food taken into the organism and the organic substances of 

 which the tissues are composed are consumed in the body by the 



