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, 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, will, as we have seen, be 
the same as that of 5 X 2 = 10 per cent of CO, 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, poisoning in the piecHee 
of abundance of oxygen. 
I must now turn to another line of investigation in 
relation to the regulation of breathing. In 1868 
