540 THE RESPIKATORY EXCHANGE 



minimum of oxygen and a maximum of carbon dioxide in the 

 alveolar air ; the converse will be the case at the end of a normal 

 inspiration. Upon these principles Haldane has based a simple 

 but efficient method of collecting samples of the alveolar air of 

 man, and in conjunction with Priestley has carried out an im- 

 portant series of experiments upon the carbon dioxide in the 

 alveolar air under different conditions and its relation to the 

 regulation of breathing. 



The following figure (Fig. 31) shows the construction of the 

 apparatus used for obtaining the alveolar air. The subject of 

 the experiment expires at the end of a normal inspiration quickly 

 and deeply through the mouthpiece and closes it with his tongue, 



Mouthpiece 



Sampling Tube 



FIG. 31. Apparatus for Collection of a Sample of Alveolar 

 Air (Haldane and Priestley}. 



The tube is a piece of stiff rubber tube about 4 feet long and 

 about 1 inch diameter. The sampling tube is completely exhausted 

 by a gas-pump before the experiment. 



until a sample of the air has been taken by the opening and closing 

 of the previously exhausted sampling tube. A second sample is 

 taken, when the subject expires deeply at the end of a normal 

 expiration. The mean of the analyses of the two samples repre- 

 sents the average composition of the alveolar air. 



Haldane and Priestley ( 21 ) found that during rest and under 

 normal atmospheric pressure the percentage of carbon dioxide in 

 the alveolar air is almost constant for the same individual ; in 

 their own cases the average percentages were 5-62 and 6- 28, the 

 maximal variations being 5-40 and 5- 87 in the former and 5-985 

 and 6-845 in the latter. Within the limits of atmospheric pressure, 

 646-1260 mm,, which were investigated, the percentage of carbon 



