SEC. 4. THE ENERGY OF THE BODY. 



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. 



The Income of Energy. 



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 

 in it, 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 grm. 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 



