CHAPTER XXI. 

 THE ENERGY EQUATION. 



We come now to a brief consideration of one of the most 

 important questions connected with the whole animal chem- 

 istry and this is the question of the liberation of energy from 

 the consumption of various foods. The function of the food 

 we eat is a multiple one. It may not only increase the body 

 weight and maintain the various functions of the body through 

 oxidation, but in its combustion heat is liberated to maintain 

 also the body temperature, and energy is furnished to enable 

 us to perform external work. It is interesting to measure the 

 effect of the food in these several directions, which may be 

 done with a fair degree of accuracy. The following consid- 

 erations will show the basis of the calculations. 



POTENTIAL ENERGY OF FOOD. 



The food consisting essentially of combustible substances is 

 the source of a large amount of potential energy. In the com- 

 plete combustion of the fats, carbohydrates and proteins of 

 the food a large amount of heat is liberated and this in turn 

 is the equivalent of a certain amount of work. The potential 

 energy of chemical substances may be measured in various 

 ways, but for purposes like the present it is customary to meas- 

 ure this energy in terms of the units of heat liberated in the 

 combustion of the body in question with oxygen. Certain 

 units are in common use : 



Unit of Heat, Calorie. The unit of heat or calorie may 

 be defined as the quantity of heat required to raise the tem- 

 perature of a gram of water one centigrade degree, at a mean 

 temperature. As the heat absorption of a gram of water is 

 not quite the same throughout the scale, the calorie is perhaps 



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