160 CARBON DIOXIDE IX EXPIRED ALVEOLAR AIR, 



forcible expiration unless this is unduly prolonged. Even when 

 much deeper expirations were made, which occupied two seconds, 

 it still remained impossible to detect any increased tension of 

 carbon dioxide in the air last expired. In the light of these 

 results, it seems unlikely that the expired air remaining in the 

 "dead space" differs in composition from that last expelled from 

 the mouth or nose during a quick, forcible expiration. Whether 

 the air was forcibly expelled at the end of an inspiration, after 

 deep breathing or after shallow respiration, the same results were 

 obtained. The last 625 c.c. expired appeared to be of uniform 

 composition. The bearing of these results on the conclusion of 

 Krogh and Lindhard, that the percentage of carbon dioxide in 

 the alveolar expired air is not identical with that in the pulmon- 

 ary alveoli, will be discussed in a later paper dealing with 

 alveolar expired air. 



Conchisions. 



(1.) The forcible expulsion of the breath into a straight brass 

 tube of 20 mm. bore ser\ es to wash out the whole of the air from 

 two metres of the tube. 



(2.) Analysis of the last 625 c.c. of expired air shows that the 

 difference in percentage of carbon dioxide, in any portion of it, 

 does not exceed the error in the analytical method, when the 

 expulsion is performed within two seconds for amounts of two 

 litres and over, and within one second for quantities of one to 

 two litres of expired air. 



(3.) When the air is expelled more slowly, the successive por- 

 tions of expired air continue to show higher percentages of 

 carbon dioxide. 



Finally, I wish to thank Sir Thomas Anderson Stuart, in 

 whose laboratory this work has been carried out, and Dr. H. G. 

 Chapman for the advice and assistance which he has g'iven me 

 in the course of this research. 



