NORMAL PROCESSES OF ENERGY METABOLISM 559 



(and the possibility of this reaction lias never been disproved) the pro- 

 duction of carbon dioxid in proportion to the amount of heat disengaged 

 would be very small and the thermal quotient would be correspondingly 

 high. Lefevre(^) has brought together results from Atwater and Bene- 

 dict's work to show that the weight of CO 2 produced for each 100 Cal. of 

 heat eliminated from the human body is very variable. The results are 

 given in the table below. 



TABLE 5 'ib'-i 



VARIATION IN HEAT EQUIVALENT OF CO 3 (ATWATER AND BENEDICT) 



Even in experiments of long duration it is evident that the calculation 

 of heat production upon the basis of the carbon dioxid contains an inherent 

 error of as much as 25 per cent. In experiments of short duration the 

 error would be even greater. In fact, of the series of experiments from 

 which the figures given above were obtained many were performed in two 

 hour periods so that it is possible to follow the heat as measured and the 

 CO 2 from period to period. In spite of a perfectly uniform heat elimina- 

 tion the CO 2 elimination varies at times as much as 40 per cent. 



c. The Respiratory Quotient and Its Significance. Even though the 

 value of the oxygen absorption in terms of heat may be fairly constant, 

 so that for long periods the calculation of the energy production may 

 proceed upon this basis with involvement of very slight error, the re- 

 quirements of short experiments are more rigorous. For it is quite pos- 

 sible that an observation of, say, only 15 minutes duration made perchance 

 soon after a meal would coincide with maximum absorption of carbohy- 

 drate; while another made some hours later might very well coincide with 

 the maximum absorption and combustion of fat. Two such periods could 

 not be concordant if the average thermal quotient for oxygen were used. 

 The respiratory quotient, however, enables us to know what kind of food 

 is being oxidized at any given time, or at least what possible combinations 

 of combustion there may be. 



If a sample of pure food, e. g., cane sugar, be placed in a bomb with 

 oxygen and ignited, it is possible to learn the amount of combustion by 

 analyzing the gases before and after firing. In the case of pure carbo- 

 hydrate it would be found that just as much oxygen by weight has disap- 

 peared as is contained in the carbon dioxid formed. Or, since equal vol- 

 umes of all gases contain the same number of molecules at the same pres- 



