128 LIFE AND DEATH. 



It is this combustion that wc have known since the 

 days of Lavoisier to be the source of animal heat. 

 We can easily determine the quantity of heat left by 

 albumen passing into the state of urea, and by the 

 starch, the sugars, and the fats reduced to the state of 

 water and carbonic acid. This quantity of heat does 

 not depend on the variety of the unknown inter- 

 mediary products which have been formed in the 

 organism. Berthelot has shown that this quantity of 

 heat which measures the chemical energy liberated by 

 these substances is identical with the quantity ob- 

 tained by burning the sugar and the fats in a chemical 

 apparatus, in a calorimetric bomb, until we get carbonic 

 acid and water, and by burning albumen till we get 

 urea. This result is a consequence of Berthelot's 

 principle of initial and final states. The liberated 

 heat only depends on the initial and final states, and 

 not on the intermediary states. The heat left in the 

 economy by the food being the same as that left in 

 the calorimetric bomb, it is easy for the chemist to 

 determine it. It has thus been discovered that one 

 gramme of albumen produces 4.8 Calories, one gramme 

 of sugar 4.2 Calories, and one gramme of fat 9.4 

 Calories. We thus gather what a given ration — a mix- 

 ture in certain proportions of these different kinds of 

 foods — supplies to the organism and what energy 

 it gives it, measured in Calories. 



The calculation may be carried out to a high degree 

 of accuracy if, instead of confining ourselves to the 

 broad features of the problem, we enter into rigorous 

 detail. It is only, in fact, approximately that we have 

 reduced all foods to albumen, sugar, and fat, and all 

 excreta to water, carbonic acid, and urea. 



The reality is a little more complicated. There 



