3 8o SCIENCE PROGRESS 



of C0 2 in our alveolar air was 6*oi at normal pressure, and 

 3*53 in compressed air at 17 atmospheres absolute pressure. 

 The corresponding partial pressure of C0 2 in the alveolar air 

 was 5 "68 per cent, of an atmosphere, or 43 mm. of mercury, 

 during respiration at normal pressure, and exactly the same 

 in the compressed air. Thus the respiratory centre normally 

 responds only to variations in the partial pressure of C0 2 in the 

 blood which irrigates it. To variations in the partial pressure of 

 oxygen it is within very wide limits absolutely indifferent. 

 Only when the partial pressure of oxygen in the air inspired 

 falls below about two-thirds of the normal does the respiratory 

 centre begin to respond to want of oxygen. Quite recent experi- 

 ments by Hill and Greenwood at high pressures in air, and by 

 Boycott and myself at low pressures in a mixture of oxygen and 

 air, have shown that at an absolute pressure of 6 atmospheres 

 the alveolar C0 2 percentage was 0*9, and at o - 4 atmospheres 

 it was 15 ; while at normal pressure it was about 5*5 with the 

 same individuals. The partial pressure of C0 2 {i.e. the per- 

 centage in the moist alveolar air multiplied by the barometric 

 pressure) remained, however, quite constant. 



If C0 2 is present in the air inspired, the effect is to make the 

 breathing deeper, and finally also more frequent ; unless, how- 

 ever, the pressure of C0 2 in the inspired air begins to approach 

 the normal alveolar C0 2 pressure, there is very little change in 

 the alveolar C0 2 pressure, compensation being produced very 

 easily. 



How, now, does this apply to the case of the diver ? On 

 account of the work involved in pumping, a diver has to content 

 himself with a minimum supply of air. Let us suppose that 

 when he is just under water he has sufficient air to prevent the 

 C0 2 percentage in the helmet air from rising beyond 3 while 

 he is at work. This will keep him fairly comfortable. If he 

 now goes down 10 metres, the air supply being the same, the 

 percentage of C0 2 in the helmet air will also remain the same. 

 As, however, the pressure is now 2 atmospheres, his normal 

 alveolar C0 2 percentage will be 2'8 instead of 5*6. It will, 

 however, be quite impossible for him to maintain this percentage 

 of C0 2 in his alveoli, since during work the percentage in the 

 inspired air itself is 3. He will thus suffer from severe dyspnoea. 

 A very little consideration will show that the diver must have 

 twice as much air at 10 metres if he is to be as comfortable as he 



