CHAPTEK XLII 



THE CONSERVATION OF ENERGY 



THE nutrition of the body has been considered in the preceding pages 

 from the standpoint of a detailed examination of the fate of the 

 various foodstuffs which enter the body from the alimentary canal. 

 Furthermore, by a consideration of the substances which the body 

 excretes, an attempt has been made to arrive at some understanding 

 of the processes of metabolic activity. 



The knowledge thus obtained is of more than theoretical interest. 

 It throws much light on one of the most important subjects which con- 

 fronts the physiologist, namely, the suitability of various substances 

 as articles of diet. This subject we propose to discuss in the present 

 chapter, but before doing so we must lay down two propositions. 



(1) A suitable diet must provide at least as much of each chemical 

 element as is excreted from the body. 



(2) The daily food must supply a store of potential energy which 

 shall equal the actual energy dissipated in the twenty-four hours. 



The first of these propositions is self-evident ; the second, which 

 resolves itself into an enquiry as to whether the living body obeys 

 the law of the conservation of energy, has been the subject of much 

 laborious research. 



In the cruder investigations of earlier workers (Lavoisier, etc.), a 

 considerable discrepancy appeared between the actual potential 

 energy of the food taken in, and the proven actual energy which is 

 dissipated by the body. More exact methods, especially in the 

 hands of Eubner, have gone far to put the energy changes of living 

 matter on a more intelligible basis, while investigations undertaken 

 during the last two decades, under the auspices of Atwater, Benedict, 

 and their colleagues, have finally established that the law of conser- 

 vation of energy holds in relation to the animal body. 



Among the forms which energy derived from the combustion of 

 any substance, whether within or without the body, may assume, two, 

 namely, mechanical work and heat, demand the attention of the 

 physiologist. The simplest case which can present itself, that in 



