68 



COMPARISONS OF RESPIRATORY EXCHANGE. 



sample is analyzed. By supporting the tube and valves on a light 

 framework placed upon the head and resting the bag upon a second 

 frame on the back, the respiration apparatus may be carried quite 

 easily a considerable distance. The accessory apparatus required for 

 this method of determining the respiratory exchange are a meter for 

 measuring the gas collected in the bag, samplers for collecting the 

 samples of air, and a gas-analysis apparatus. 



In experimenting the bag is placed in a suitable position and a sup- 

 port arranged for the valves and tubing. The subject then inserts 

 the mouthpiece and commences respiration, with the three-way valve 

 so turned that the ah* expired passes out into 

 the surrounding atmosphere. After equilibrium 

 of respiration has been established, the three- 

 way valve is turned so that the expired air will 

 enter the bag. The experiment is then con- 

 tinued the determined length of time, this being 

 limited by the size of the bag used and the kind 

 of experiment. After the experiment is ended, 

 the gas in the bag, when thoroughly mixed, 

 is forced through a meter, the barometric 

 pressure and the temperature of the meter being 

 recorded. A sample of the gas is also taken for 

 analysis. The bag should be emptied com- 

 pletely, which can be done by rolling it up when 

 nearly empty and allowing it to flatten naturally. 

 This process for expelling the air should like- 

 wise be used before the experiment in order to 

 insure the same residual volume as at the end 

 of the experiment. The rate of diffusion 

 through the wall of the bag must be deter- 

 mined by analysis, as a bag allowing any 

 determinable escape of carbon dioxide during 

 the carrying out of a respiration experiment 

 can not be used. The tests can be made by filling the bag with ex- 

 pired air and taking samples for analysis at such intervals as will 

 correspond with the length of time the expired air ordinarily remains 

 in the bag. 



In using the Douglas method in this research, two bags were em- 

 ployed. One of these a gas bag of practically pure gum was sup- 

 posed to contain 100 liters, but without appreciable pressure would not 

 hold more than 20 to 30 liters. The other bag was the largest used by 

 Douglas and was capable of containing 100 liters. This was made to 

 order of heavy rubber cloth according to measurements given by Douglas 

 in a private communication. A 10-liter Bohr meter was used for measu- 

 ring the gas in the bag. Samples of the air were collected over mercury 



FIG. 29. Mica-flap valve 

 used with the Douglas 

 method. 



The valve is shown with 

 a portion cut away so that 

 the interior is seen. The 

 direction of the air-current 

 is from A to E and is deter- 

 mined by the movements of 

 the mica flap C, the cross- 

 wires D, D, keeping the flap 

 in place. 



