18 COMPARISONS OF RESPIRATORY EXCHANGE. 



the outgoing air-pipe at a point near its exit from the chamber. The 

 psychrometer consists of a dry thermometer and wet thermometer 

 arranged with their bulbs inserted air-tight in the outgoing pipe. 

 The wet bulb is kept moist by means of a thin layer of fine linen wrapped 

 around the bulb, with its lower end dipping in a reservoir of water situ- 

 ated in a depression in the air-pipe. Both thermometers are placed 

 near the front opening, so that they may be read with a lens to 0.01 C. 

 from outside of the chamber. Readings are taken during the last 

 5 minutes of the period. The carbon-dioxide content is determined by 

 the analysis of a sample obtained by diverting a small current of the 

 outgoing air through a glass sampler. This glass sampler is connected 

 by rubber tubing between the mercury trap and the pipe leading to 

 the ingoing air-current. Air runs through the sampler during the 

 whole period. At the conclusion of the period the ends of the tube are 

 closed, the sampler taken off, and another put in its place. The sample 

 of air is then analyzed for carbon dioxide by means of the Sonden 1 

 gas-analysis apparatus. 



The temperature of the apparatus is obtained from the measurement 

 of the changes in resistance of a set of thermometers placed at five 

 different points in the chamber, approximately 2 or 3 cm. from the wall. 

 Their general construction is described in detail in a former publica- 

 tion. 2 The barometer readings were obtained from a brass-scale mer- 

 cury barometer equipped with a vernier reading to 0.05 mm. 



The carbon-dioxide production of a subject for an experimental 

 period is obtained from the increase in weight of the soda-lime container 

 and the sulphuric-acid container following it, plus or minus the changes 

 in carbon-dioxide content of the air of the apparatus. 



The oxygen consumption of the subject for an experimental period 

 is obtained from the loss in weight of the cylinder corrected for the 

 changes in oxygen content of the apparatus and the admission of nitro- 

 gen and argon with the oxygen. The change in the residual content of 

 oxygen is calculated from the total volume of the air corrected to 

 C. and 760 mm. by subtracting from it the volume of carbon dioxide 

 and water- vapor in the chamber at the end of the period. The volume 

 of the nitrogen in the apparatus remains constant, except for the small 

 amount present in the oxygen admitted from the cylinder, a correction 

 being made for this on the loss in weight of the cylinder. 3 



The general routine of an experiment with the apparatus is as 

 follows: When the subject is ready for the experiment a stethoscope is 

 attached to the chest, and, in some instances, an electrical thermometer 

 is inserted in the rectum. A pneumograph is also sometimes placed 



1 A detailed description of the most recent form of this apparatus is given by Benedict in Car- 

 negie Inst. Wash. Pub. 166, 1912, p. 76. 



2 Benedict and Carpenter, Carnegie Inst. Wash. Pub. 123, 1910, pp. 28-29. 3 Ibid., pp. 84 and 88. 



