May, 1929] SIMPLIFIED TECHNIQUE FOR MEASURING ENERGY 17 



are not set in motion, the oxygen content in the chamber air will decrease 

 below that of outdoor air, and the carbon dioxide content will rise above it, 

 so that in case of a full-fed animal the former may decrease to one per cent 

 below and the latter may increase to one per cent above the outdoor 

 conditions within half an hour. It will thus be observed that while the 

 oxygen supply is not materially exhausted, the carbon dioxide content 

 does increase above the outdoor or fresh condition in a relative sense. 

 Contrary to common opinion, such increase in carbon dioxide approaching 

 one per cent is not injurious or disturbing. It is thus an advantage to let 

 the animal build up the carbon dioxide content of the chamber air to from 

 0.60 to 0.80 per cent before starting ventilation and then to maintain the 

 rate of ventilation at a speed whereby the removal of air from the chamber 

 maintains the percentage of carbon dioxide in the chamber at an even 

 level. This minimizes the probable error in gas analysis, particularly in 

 the calculation of the oxygen deficit. 



Gas Analysis 



The experiment is thus always begun when the animal has already 

 materially altered the composition of the air which is residual in the 

 chamber. When the experiment is ended there will remain in the cham- 

 ber some carbon dioxide which the animal has contributed during the 

 experiment but which has not been collected. This makes it necessary to 

 analyze samples of the chamber air taken both at the moment of starting 

 and of ending the experiment, in order to determine the amount of oxygen 

 and of carbon dioxide which was in the chamber when the experiment was 

 begun and the amount left over when it ended. These determinations 

 enable one to strike a balance for the amount of oxygen consumed and 

 carbon dioxide produced during the specific time interval of the experi- 

 ment. 



The great improvements which have recently been made in apparatus 

 for gas analysis have made possible a much broader and more thorough 

 study of metabohsm and thus have advanced the potential significance of 

 indirect calorimetry. With open-circuit apparatus gas analysis was 

 formerly carried out on carbon dioxide in order to provide a proper bal- 

 ance for the amount absorbed. With the newer types, such as the Car- 

 penter gas analysis apparatus,^ with which both carbon dioxide and 

 oxygen can be determined rapidly and with an extraordinary degree of ac- 

 curacy (one part in 100,000), the respiratory quotients (that is, the ratio 

 of the two gases) may now be obtained at any time. Thus, one may not 

 only determine the amount of energy expended during a given period, but 

 one may also follow the course of digestion and show the character of the 

 material that is being oxidized, or burnt, in the body at any given time. 



The apparatus used here (see Plates 3-4) are of the most recent type 

 iesigned by Carpenter. Two of these, which are dupHcates (Plate 4, 

 "ight), allow both oxygen and carbon dioxide determinations. The 

 raising and lowering of the mercury level is performed mechanically by an 

 3lectric motor. The arrangement is such that one or both apparatus may 

 De operated at the same time by one person. These apparatus are pro- 

 vdded with large reservoirs for the absorbing fluids, and the manner of 



1 Carpenter, T. M., Journ. Metab. Research, 1923, 4, p. 1; Carpenter, T. M., and E. L. 

 Fox, Journ. Biol. Chem., 1926, 70, p. 115; Carpenter, T. M., Journ. Biol. Chem., 1929, 

 n press. 



