REGULATION OF BREATHING 19 
rest; but during any exertion the effect is not merely 
noticeable, but very trying. During moderate work 
in pure air the breathing is three or four times what 
it is during rest; but when air containing 3 per cent 
of CO, is breathed the increase is to 6 or 8 times the 
amount of air breathed during rest in pure air. Pant- 
ing is thus very severe, and hinders all hard work. 
Constant employment on hard work such as mining 
in air of this composition is apt to produce in the lungs 
the condition known as emphysema, and thus to cause 
premature disablement. The ventilation of a mine 
ought, therefore, to be at least sufficient to prevent 
the CO, percentage from exceeding about 1 per cent, 
where no other gaseous impurities than CO, are to 
be found. 
One of the most interesting examples of the effects 
of CO, is that which occurs in diving with the ordi- 
nary diver’s equipment. The diver is supplied with 
air by a pipe through which air is pumped down to 
him. The air passes into his helmet, and escapes into 
the water by a valve situated at the side of the helmet. 
The deeper he goes the greater is of course the pres- 
sure at which this air must be supplied; and the com- 
position of the air which he breathes in the helmet 
will of course depend on the amount of air supplied 
to him and on the rate at which he vitiates this air. 
During work, for instance, he may produce four or 
five times as much CO, as during rest, so that he will 
need correspondingly more air during work. 
Supposing that the diver is working at a depth of 
22 fathoms, or 132 feet, the air supplied to him will 
