20 ORGANISM AND ENVIRONMENT 



have a total barometric pressure of five atmospheres. 

 If, now, the rate of supply, as measured by the strokes 

 of the pump, is such as would keep the percentage of 

 CO 2 in the air of the helmet at not more than 2 per 

 cent during work, this quantity of air would suffice 

 to keep him comfortable if he were at or near the sur- 

 face. But if the same quantity of air is supplied to 

 him at 22 fathoms, or five atmospheres' pressure, the 

 effect of 2 per cent of CO 2 will, as we have seen, be 

 the same as that of 5 X 2 = 10 per cent of CO 2 at 

 surface. Hence if the diver exerts himself he will not 

 merely pant excessively, but rapidly lose conscious- 

 ness. It used to be a common occurrence for divers 

 to lose consciousness in this way; and the fact that 

 British naval divers were so commonly unable to do 

 any work at considerable depths led to an investiga- 

 tion of the whole subject in the light of the new knowl- 

 edge available, and to the laying down of regulations 

 which now make work quite easy at the greatest depths 

 required. The air supply to a diver ought evidently 

 to be increased in direct proportion to the increase in 

 the atmospheric pressure at which he works. 



A diver is in no danger from want of oxygen, since 

 the pressure of oxygen in his helmet and in his alveo- 

 lar air is always far higher than in pure air at surface. 

 It is almost always from oxygen want that a man 

 dies who is asphyxiated by vitiated air in mines; but 

 a diver may die from CO 2 poisoning in the presence 

 of abundance of oxygen. 



I must now turn to another line of investigation in 

 relation to the regulation of breathing. In 1868 



