BY H. S. HALCRO WARDLAW. 789 



Recently, Boothbv and Sandiford (J 916) have also used the 

 above aerotonoraetric method for the determination of the venous 

 tension of carbon dioxide, and have obtained results similar to 

 those of Christiansen, Douf,das, and Haldane {loc. cit.). 



In the present investigation, the rate at which the cotnposition 

 of alveolar air changes, when the admission of fresh air to the 

 lungs is discontinued, has been examined in greater detail. The 

 rate of alteration of the composition of the alveolar air has been 

 studied under two different sets of conditions. In the first 

 series of experiments, the breath was simply held for measured 

 periods after the completion of a normal inspiration, before 

 taking a sample of alveolar air. In the second series of experi- 

 ments, after the completion of a normal inspiration, breathing 

 was continued into and out of an empty rubber bag, samples of 

 alveolar air being collected from time to time. It was found 

 that the rise in the alveolar tension of carbon dioxide and the 

 fall in the tension of oxygen were considerably more rapid in 

 the second series of experiments than the first. 



Experiments have also been carried out to examine more closely 

 the difference between the two sets of results. It was found 

 that movement of the same air into and out of the lungs, altera- 

 tion of pressure in the closed chest, or the maintenance of nega- 

 tive pressure in the chest, caused a marked increase in the rate 

 of increase of the alveolar tension of carbon dioxide. The main- 

 tenance of positive pressure in the chest, however, slightly de- 

 creased the rate of change of composition of the alveolar air 

 after the cessation of normal respiration. 



Methods. 



The experiments described in this paper were made upon one 

 subject (H.S.H.W.). Before commencing to collect samples of 

 alveolar air, the subject seated himself comfortably and rested 

 for ten minutes in order to allow the respiration to become as 

 steady as possible. The subject remained seated during the 

 whole course of an experiment. The only work done by him 

 was the opening of taps for the collection of samples of alveolar 

 air, the starting and stopping of a kymograph, and the making 



