SEC. 2. THE ENERGY OF THE BODY. 



The Income of Energy. 



421. Broadly speaking, the animal body is a machine for 

 converting potential into actual energy. The potential energy 

 is supplied by food ; this the metabolism of the body converts 

 into the actual energy of heat and mechanical labour. We have 

 in the present section to study what is known of the laws of 

 this conversion, and of the distribution of the energy set free. 



Neglecting all subsidiary and unimportant sources of energy, 

 we may say that the income of animal energy consists in the 

 oxidation of food into its waste products, viz. the oxidation of 

 proteids, fats and carbohydrates into urea, carbonic acid and 

 water. A principle laid down by the chemist teaches that the 

 potential energy of any body, considered in relation to any 

 chemical change which it may undergo, is the same when the 

 final result is the same, whether that result be gained at one 

 leap or by a series of steps ; that, for instance, the energy set 

 free by the oxidation of 1 grin, of fat into carbonic acid and 

 water is the same, whatever the changes forwards or backwards 

 which the fat undergoes before it finally reaches the stage of 

 carbonic acid and water; and similarly, that the energy available 

 for the body in 1 grm. of dry proteid is the energy given out 

 by the complete combustion of that 1 grm., less the energy 

 given out by the complete combustion of that quantity of urea 

 to which the 1 grm. of proteid gives rise in the body. Taking 

 this as our guide we can readily calculate the amount of poten- 

 tial energy contained in an average 24 hours' diet, and thus 

 obtain the average daily income of energy. For the potential 

 energy of most of the substances used as food has been deter- 

 mined by direct calorimetric observations ; and the several 

 determinations, though they vary somewhat, agree sufficiently 

 closely to serve as data for the calculations in question. 



The total combustion of the following substances has given 

 for one gramme of each substance the following results expressed 

 in calories, that is in gramme-degree units of heat. 



632 



