BED RESPIRATION CALORIMETER. 17 



magnet and the movement of its armature opens or closes the tube 

 leading from the reduction valve on the cylinder to the chamber. 

 The spirometer not only regulates the admission of the oxygen, but also 

 provides for sudden changes in the air volume due to changes in the 

 temperature of the air in the chamber or to changes in barometric 

 pressure. 



When a subject is breathing in the apparatus, it is not sufficient 

 simply to weigh the absorbers and the oxygen cylinder in order to 

 determine the amounts of carbon dioxide and water-vapor exhaled 

 and the oxygen consumption in any given length of time, for the actual 

 carbon-dioxide and water-vapor content of the air in the chamber 

 may vary from time to time; the actual oxygen content may also vary 

 because of variations in temperature and pressure as well as variations 

 in amounts of carbon dioxide and water-vapor. Accordingly, the 

 amounts of carbon dioxide and water-vapor in the air residual in the 

 chamber should also be determined at the beginning and the end of 

 the experimental period. At the same time a measurement of the 

 temperature of the air in the apparatus should be made and the barom- 

 eter read. 



The water-vapor and carbon dioxide in the air-current were formerly 

 determined in the following manner: A portion of the outcoming air 

 was diverted at a point just before its entrance into the first sulphuric- 

 acid container. A mercury trap, D, shown in figure 1, served for open- 

 ing and closing the branch tube. When the leveling bulb, E, was 

 lowered, the mercury flowed away from the U-tube D and allowed the 

 air to pass through it. A small tube led from D to a set of three 

 U-tubes, A, containing sulphuric acid and pumice stone, soda-lime, and 

 sulphuric acid and pumice stone, respectively. The exit tube of the 

 last U-tube was connected with a 10-liter Bohr meter. 1 From the 

 Bohr meter a tube led to a drying-tower and then to the ingoing air- 

 pipe. The carbon dioxide and water-vapor of the outcoming air were 

 determined by lowering the mercury level and passing 10 or 20 liters 

 of air through the weighed U-tubes and meter, then raising the bulb 

 again. The increases in weight of the U-tubes gave the amounts 

 absorbed from the volume of air as indicated by the readings of the 

 meter. The determination took place during the last 10 or 15 minutes 

 of the experimental period. 



In the winter season of 1911-12 another method of determining 

 the carbon-dioxide and water- vapor content of the outcoming air was 

 devised by Professor Benedict and used thereafter. According to this 

 method, the water- vapor content is determined by calculation from the 

 readings of a psychrometer 2 installed inside the respiration chamber in 



^ee A-B, fig. 1, p. 15. 



This psychrometer is described by Benedict and Talbot in Carnegie Inst. Wash. Pub. 201, 

 1914, p. 37. 



