WORK UNDER PRESSURE AND IN GREAT HEAT 379 



ship : the air escapes through an adjustable spring valve at the 

 side of the helmet. The arrangement is thus such that the 

 pressure of the helmet air breathed by the diver is always equal 

 to, or slightly greater than, the pressure of the water at the 

 valve outlet. At a depth of 10 metres (33 ft.) the diver is there- 

 fore breathing air at an excess pressure of 1 atmosphere, 

 or at an absolute pressure of 2 atmospheres ; and every 

 additional 10 metres will add another atmosphere to the 

 pressure. 



Provided that about 50 to 100 litres of air per minute are 

 supplied to the diver, and that the outlet valve of his helmet is 

 properly adjusted, he can work quite comfortably at depths 

 up to about 20 metres. Beyond this depth, however, he usually 

 feels increasing discomfort and difficulty in working ; so that 

 at 40 or 50 metres work becomes extremely difficult. This 

 difficulty has hitherto been ascribed to the pressure : its real 

 cause was one of the first points which we investigated. We 

 found that the discomfort of the diver was associated with 

 laboured or dyspnceic breathing ; and this fact, in connection 

 with the results of recent experiments on the normal regula- 

 tion of breathing, suggested the true explanation. 



In a paper published two years ago by Mr. Priestley and 

 myself, 1 it was shown that under normal conditions the breath- 

 ing is always regulated in such a way as to keep the partial 

 pressure of the carbon dioxide in the air of the lung alveoli 

 almost exactly constant. This fact was established by analysing- 

 samples of air from the lung alveoli in man. To obtain a 

 sample all that is necessary is to make a sudden deep expira- 

 tion (during perfectly natural breathing) through a long piece 

 of tubing of wide bore, and at once withdraw into a gas- 

 analysis apparatus a portion of the air left in this tube at the 

 end of expiration. The deep expiration washes out all the 

 more or less pure air contained in the air-passages and tube 

 before the deep expiration, and leaves the tube full of pure 

 alveolar air. This pure alveolar air contains for each indi- 

 vidual an almost astoundingly constant percentage of C0 2 if 

 the barometric pressure is constant. If the barometric pressure 

 varies, the percentage of C0 2 varies inversely as the barometric 

 pressure, so that the partial pressure of C0 2 remains constant. 

 For instance, we found in ourselves that the mean percentage 



1 Journal of Physiology, vol. xxxii. p. 225, 190^. 



