542 THE PHYSIOLOGY OF ALIMENTATION. 



ray and torpedo. Finally luminous energy may be produced from vital 

 energy, as is the case with the phosphorescent animals. 



It is useless to weaken principles by thus enumerating all the restric- 

 tions which attend them. It is well known that there are no absolute 

 natural principles. It is sufficient to say that the energy which tem- 

 porarily animates living creatures is furnished to them from the exter- 

 nal world exclusively in the form of potential chemical energy, and that 

 it returns to the outer world chiefly in the form of heat but partially in 

 the accessory form of mechanical energy. 



It is clear that if the flow of energy which circulates through the 

 animal leaves it solely in the form of heat, then this heat becomes a 

 possible measure of the amount of energy originally furnished by the 

 aliment. If the outward flow is divided between two channels, heat 

 and mechanical work, the two amounts of energy thus given up must 

 be added together. In the case where the product is heat alone we 

 need only to determine the loss of heat by the calorimeter to have a 

 measure of the consumption of energy in living. Physiologists have 

 arranged apparatus in various forms for this determination. Lavoisier 

 and Laplace employed the ice calorimeter. They placed an animal of 

 small size in an ice cage and determined the amount of heat given out 

 by the amount of ice melted. In one of their experiments they found 

 that the Indian pig melted 341 grams of ice in ten hours and conse- 

 quently furnished 27 calories of heat. 



More recently a better instrument has been devised. M. d'Arsonval 

 employed an air calorimeter, which is nothing but a differential ther- 

 mometer very ingeniously constructed and made self-registering. 

 Rosenthal, Eichet, Hirn andKaufmann, and Lafevre have used air calori- 

 meters more or less complex. Others, following the example of Dulong 

 and of Despretz, have used water and mercury calorimeters, or like 

 Liebermeister, Winternitz, and Lefevre have had recourse to the method 

 of baths. There have been many of these researches and they have 

 contributed very interesting results. 



The same problem may be solved in another way. Instead of deter- 

 mining the energy leaving the body in the form of heat it may be meas- 

 ured before its entrance in the form of chemical potential. This deter- 

 mination has been made in the same units as the preceding — that is to 

 say, in calories. It has been owing to the advances in thermo-chemistry 

 and to the principles advanced in 1864 by Berthelot that this second 

 method of arriving at the energetic equivalent of nutrition has become 

 possible. Physiologists by the aid of these methods have established 

 the balance of energy for the living being in various conditions, as they 

 had already done before for matter. If it is asked what has been the 

 outcome of these researches, we reply that it consists in having deter- 

 mined an enormous mass of separate facts, of which we can not here 

 speak, but which have served to build up the general doctrine of the 

 energetics of biology — that fertile conception which enables us to 



