142 LIFE AND DEATH. 



duces exactly the amount of food which gives off 

 sensibly this number of Calories. Now, this is an 

 interesting fact, but, like the preceding, it has no 

 demonstrative force. 



Objections. T/ie Limits of Isodynamism. — On the 

 contrary, there are serious objections. The thermal 

 value of the nutritive principles only represents one 

 feature of their physiological role. In fact, animals 

 and man are capable of extracting the same profit 

 and the same results from rations in which one of the 

 foods is replaced by an isodynamic proportion of the 

 other two — that is to say, a proportion developing the 

 same quantity of heat. But this substitution has very 

 narrow limits. Isodynamism — that is to say, the 

 faculty that food has of supplying /r^ rata its thermal 

 values — is limited all round by exceptions. In the 

 first place, there are a few nitrogenous foods that no 

 other nutritive principle can supply ; and besides, be- 

 yond this minimum, when the supply takes place, it is 

 not perfect. Lying between the albuminoids and the 

 carbohydrates relatively to the fats, it is not between 

 these two categories relatively to nitrogenous sub- 

 stances. If the thermal power of food were the only 

 thing that had to be considered in it, the isodynamic 

 supply would not fail in a whole category of principles 

 such as alcohol, glycerine, and the fatty acids. Finally, 

 if the thermal power of a food is the sole measure of 

 its physiological utility, we are compelled to ask why 

 a dose of food may not be replaced by a dose of heat. 

 External warming might take the place of the internal 

 warming given by food. We might be ambitious 

 enough to substitute for rations of sugar and fat an 

 isodynamic quantity of heat-giving coal, and so 

 nourish the man by suitably warming his room. In 



