The determination of the alveolar air tension was made with a modified form of the Haldane apparatus on samples of alveolar air obtained by a method previously described.1 In this method of collection a 3-way valve is used which is attached to a 6-foot length of rubber tubing and a mouthpiece. The subject at first breathes through the short arm of the valve, and is cautioned to breathe normally and no deeper than usual and to hold the lips firmly about the mouthpiece. While he breathes through the side opening of the valve the operator watches the rise and fall of the chest or abdomen. At the end of an expiration, the time is noted and the valve is turned to such an angle that the subject breathes quietly into the long rubber tube. After 20 seconds and at the end of an expiration, the valve is turned back quickly to the original position. At the usual respiratory rate, 20 to 22 seconds will allow for 4 or 5 respirations. A small light feather fastened across the outlet of the long tube will indicate by its movements the end of the expiration. After the valve is turned, the samples are quickly drawn from the rubber tube through an aperture near the valve and analyzed as usual by the Haldane method. Prior to the second test, the long tube is thoroughly ventilated to expel the residual air from the preceding respirations. The results for each test are based on the average of the analysis of two samples taken successively.

1Roth, Journ. Am. Med. Asao., 1915, 65, p. 413; also Boston Med. and Surg. Journ., 1918, 179, p. 130.