516 JOHN K. MUELIN 



milligram-calories in ten hours, while the highest accuracy attainable by 

 Bohr and Hasselbalch with their egg calorimeter was 100 milligram-calo- 

 ries. The percentage difference is not so great as this in applying the two 

 methods simultaneously to the study of the human organism; but one 

 comes very soon to rely upon the indirect measurement more than the 

 direct (see page 5SG). Furthermore, and in the third place the two meth- 

 ods agree very closely in the best forms of respiration calorimeters. This 

 being true and the indirect method being both simpler and more reliable, 

 greater space will be given to its description and to the methods of calcu- 

 lating energy production from the fundamental data, than for the direct 

 method. 



I. Methods of Measuring the Respiratory Exchange 

 by Means of a Respiration Chamber 



The methods of measuring respiratory metabolism are of two general 

 kinds: (a) one requiring a chamber in which the subject is confined, and 

 (b) a method so devised that the respiratory passages are connected di- 

 rectly with the measuring apparatus. 



Two general types of ventilation also have been used, one known as the 

 open-circuit and the other as the closed-circuit type. The classical instance 

 of the first type is the apparatus of Pettenkofer first described in 1863 and 

 later improved by C. Voit. The classical instance of the closed-circuit type 

 is the Regnault-Reiset apparatus first described in 1849. Only the more 

 important constructions of each type will be described here. 



1. Open-circuit Type of Apparatus. a. Pettenkofer Apparatus. 

 The original apparatus of Pettenkofer consisted of a chamber containing 

 12.7 cubic meters which was ventilated by means of air pumps drawing 

 air from the outside. The air was aspirated through the chamber and at 

 the point of exit samples were measured after having been passed through 

 pumice stone saturated with sulphuric acid thence through barium hydrate 

 for the absorption of the carbon dioxid. In the earliest experiments per- 

 formed with the apparatus by Pettenkofer the efficiency of the absorption 

 system was checked by burning candles in the apparatus and an error of 

 1.96 per cent w r as found as the average for a considerable number of tests. 

 The error on the water absorption was somewhat higher, varying from 2.5 

 to 3.5 per cent. 



This apparatus was used exclusively with the human subject. For 

 obtaining the oxygen absorption Pettenkofer and Voit(c) employed the fol- 

 lowing method : Adding to the original weight of the subject the amount 

 of food consumed and the amount of water drunk a sum was obtained which 

 was subtracted from the final weight of the subject plus all of the excreta 

 (urine, feces, carbon dioxid and water vapor). The difference between 

 these two sums was taken as the oxygen absorption. 



