president's address. 29 



limits without altering the alveolar tension of carbon dioxide, 

 provided that the depth of the breathing is allowed to regulate 

 itself naturally with no forcing or holding back. Thus by vary- 

 ing the frequency, one can greatly vary the depth without alter- 

 ing the mean tension of alveolar carbon dioxide." Haldane then 

 stated how he had verified this fact upon himself with such a 

 wide range of rates of breathing as 30 and 4 per minute, 24 and 

 G per minute, and so on. In these experiments, Haldane ob- 

 tained a close agreement. We have not been able to observe 

 this constancy under the conditions of ordinary life. We feel 

 quite confident in asserting that the tension of carbon dioxide 

 in the alveolar air of persons not obviously doing work and 

 breathing naturally varies over a range of 4 mm.Hg. every few 

 seconds. We believe and are satisfied that constancy can be 

 obtained by resting on a couch for an hour, by avoiding food and 

 keeping quite still and free from excitement. With these pre- 

 cautions, the variation in the tension of carbon dioxide in the 

 alveolar expired air may be reduced to one millimetre of mercury. 



At the same time Haldane presented some figures of experi- 

 ments confirmatory of his original contention that the deeper 

 part of the expiration contains no more carbon dioxide than the 

 middle part. Haldane employed the simple method of analysing 

 a sample from the last portion of the expired breath in a series 

 of expirations varying in depth from 190c. cm. to 1350 c. cm. 

 When the expiration exceeded 650 c.cm., the averaged figures 

 showed a close agreement. If the individual figures are ex- 

 amined, the range of variation is greater. Thus, in a series, the 

 maximum and minimum figures are 5\37% and 5*63%, while the 

 average is 5 - -J8%. 



It seemed desirable to examine more carefully the composition 

 of the successive portions of the expired breath. It would be 

 more accurate to do tins upon a single breath than by comparing 

 portions of different expirations. We made use of a long brass 

 tube fitted with side-pieces at intervals. In this way it was pos- 

 sible to draw off samples from about each 100 c.cm. in the last 

 part of the expired air. Numerous experiments were performed 

 with a number of modifications in detail, to make quite certain 



