ALIMENTARY ENERGETICS. I27 



rent of energy as it emerges and in the form of 

 heat, we may try and capture it at its entry in the 

 form of potential chemical energy. 



The evaluation of potential chemical energy may 

 be effected with the same unit of measurement as the 

 preceding — that is to say, the Calorie. If we consider 

 man and mammals, for example, we know that there 

 is only apparently an infinite variety in their foods. 

 We may say that they feed on only three substances. 

 It is a very remarkable fact that all the complexity 

 and multiplicity of foods, fruits, grains, leaves, animal 

 tissues, and vegetable products of which use is made, 

 reduce to so great a simplicity and uniformity, that all 

 these substances are of three types only: albuminoids, 

 such as albumen or white of egg — foods of animal 

 origin or varieties of albumen; carbo-hydrates, which 

 are more or less disguised varieties of sugar; and 

 finally, fats. 



Here, then, from the chemical point of view, leaving 

 out certain mineral substances, are the principal 

 categories of alimentary substances. Here, with the 

 oxygen that is brought in by respiration, is every- 

 thing that penetrates thq organism. 



And now, what comes out of the organism ? Three 

 things only, water, carbonic acid, and urea. But the 

 former are the products of the combustion of the 

 latter. If we consider an adult organism in perfect 

 equilibrium, which varies throughout the experiment 

 neither in weight nor in composition, we may say that 

 the receipts balance the expenditure. Albumen, sugar, 

 fat, plus the oxygen brought in, balance quantitatively 

 the water, carbonic acid, and urea expelled. Things 

 happen, in fact, as if the foods of the three categories 

 were burned up more or less completely by the oxygen. 



