52 A RESPIRATION CALORIMETER. 



free from carbon dioxide when taken under these conditions. It was 

 found, however, that at the time when the air sample is usually taken, 

 i. e., immediately after the end of the experimental period, the air in the 

 main ventilating pipe leaving the absorbers is a mixture of purified 

 air from the respiration chamber and normal air from the laboratory 

 contained in the carbon-dioxide and water absorbers that had just 

 been put into use, and, since the percentage of oxygen in the normal 

 air is somewhat larger than that in the air from the chamber, the pro- 

 portion of oxygen in the sample would of course be too large. If the 

 taking of the sample was delayed for several minutes, i. c, , until the 

 air in the absorbing system had been thoroughly swept out, this diffi- 

 culty was no longer experienced, but, as any delay in taking the sample 

 of air was accompanied by a gradually varying percentage of oxygen, it 

 was evident that this method of taking the sample was erroneous. To 

 avoid these difficulties, the air that has been through the residual U tubes 

 and meter is now utilized as the sample. Inasmuch as this sample is 

 always taken during the second residual, i. e. , after the air in the meter 

 has been thoroughly swept out by air of the same composition as the 

 sample, the air thus collected probably represents better than any other 

 the true composition of the carbon-dioxide free air inside the chamber. 



METHOD OF SAMPLING. 



It has been found that if the analyses are made immediately after 

 drawing the sample the air may be collected in an ordinary rubber 

 foot-ball bladder. In taking the sample it is customary to slip the 

 rubber neck of the bag over the glass tube connecting with the T tube 

 (/, fig. 19). The rubber neck of the bag is provided with a screw 

 pinchcock. On opening this pinchcock and the one above, air rushes 

 into the bladder rather than through the sulphuric acid in the drying 

 bottle until the tension on the rubber bag is sufficiently great to force 

 the air again through the sulphuric acid in the drying bottle. By 

 squeezing the bag together with the hands the air can be discharged 

 again into the drying bottle and thence into the main air-pipe. In so 

 doing there is no gain or loss of air to the system. Care must be taken, 

 however, to see that the pressure on the rubber bag is not enough to 

 force the level of the water in the separating chamber B (fig. 19) down 

 to such a point that air can escape along with water through the overflow. 



It has been found by repeated tests that the amount of air contained 

 by an ordinary foot-ball bladder under the tension here used is about 

 0.80 liter, and this quantity of air is removed from the main air-circuit. 

 A correction for this amount is made in the data for the experimental 

 period from 7 a. m. to 9 a. m., in making which it is customary to 

 assume that this volume of air consists of one-fifth of oxygen and four- 



