386 THE SCIENTIFIC MONTHLY 





All these questions require precise investigation before one 

 is justified in drawing conclusions concerning them. If such 

 investigations are to be used as a basis of recommendation con 

 cerning diet in peace or of regulation of diet in war, it is essen 

 tial that the laws of energy transformation be expressed in a 

 quantitative form. 



Nutritional physiologists agree that, as far as energy is 

 concerned, the food requirements of the living organism shall 

 be expressed in calories per unit of time. Thus a physical 

 standard is taken over from the quantitative sciences of physics 

 and chemistry. Theoretically, then, the metabolism must be 

 determined by placing the subject in a calorimeter and directly 

 measuring the number of calories produced. This has been 

 done in a large number of cases. 



Since, however, the setting free of energy in the human 

 body is merely a process of combustion, the measurement of the 

 amount of oxygen consumed and the quantity of carbon dioxide 

 excreted from the lungs should furnish a good index of heat 

 production. Thus the nutritional physiologist may avail him 

 self of the method of indirect calorimetry as well as of direct 

 calorimetry. Heat production, in short, may be determined in 

 a calorimeter or it may be computed from the gaseous exchange 

 as measured in a respiration chamber. 



The development of apparatus by which the heat produc 

 tion of the living organism may be directly measured in the 

 calorimeter or by which the gaseous exchange may be precisely 

 determined in the respiration chamber has occupied the atten 

 tion of a large number of ingenious experimenters, among 

 whom may be mentioned Lavoisier, Rubner, Zuntz, Atwater, 

 Rosa, Lusk and Du Bois. The labors of these and others have 

 brought the apparatus for the measurement of both heat pro 

 duction and gaseous exchange to such a high degree of refine 

 ment that the manipulative phases of nutritional physiology 

 may be regarded as among the most exact techniques of biolog 

 ical experimentation. Extensive comparative studies hav 

 shown that, in the case of human subjects, it is much simple 

 and essentially as accurate to calculate the heat production in 

 directly from the gaseous exchange than to measure it in th 

 calorimeter. 



The problem is not, however, solely one of physical am 

 chemical measurement. A number of biological factors mus 

 be taken into account. Muscular activity and the stimulator; 

 action of recently ingested food are of chief importance. Th 

 apparatus with which students of human nutrition now wor 



