328 V-ETEEINAKY PHYSIOLOGY 



and H 2 0, gives off a certain amount of kinetic energy, and 

 the amount of energy liberated is the same whether the 

 oxidation is direct or takes place through any of many possible 

 lines of chemical change. 



In whatever ways a chemical substance breaks down into 

 certain final products, the energy set free is always the same, 

 and this principle is taken advantage of in determining the 

 energy value of the food-stuffs. If fats and carbohydrates 

 are changed to carbon dioxide and water in the body, and if 

 the energy given off as they undergo this change can be 

 measured outside the body, their energy value as foods can 

 be ascertained. 



Determination of Energy Value. This is done by burning 

 a definite quantity of the material and finding how many 

 degrees a definite quantity of water is heated. This gives 

 the energy in heat units, and, by Joule's law, it can be con- 

 verted into the equivalent for any other kind of energy, such 

 as mechanical work. 



It is known that the energy required to heat one kilo- 

 gramme of water through one degree Centigrade is sufficient 

 to raise 423 kilograms of matter to the height of one 

 metre against the force of gravity, and thus, if the energy 

 value of any material as a producer of heat is known in 

 heat units kilogram degrees or Calories by multiplying by 

 423 we get the value in work units kilogram metres 

 (kgms.). 



The apparatus used for ascertaining the heat produced by 

 the combustion of material is called a calorimeter. Many 

 different forms are in use, but the object in the water calori- 

 meter is to secure that all the heat is used in raising the 

 temperature of a known volume of water. 



The value of the three great proximate principles of the 

 food must be considered in detail. 



1. Proteins. The chemistry of these bodies has been 

 already considered (pp. 10-12). They are the "chief sub- 

 stances" of living matter, forming about 80 or 90 per cent, 

 of its dry residue. The molecule is one of great complexity, 

 and contains C, H, 0, N, S., and sometimes P and Fe. 



It is from the proteins of the food alone that the nitrogen 



