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 

 CO2 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 CO2 will, as we have seen, be 

 the same as that of 5 X ^ = 10 per cent of COg 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 CO2 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 



