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 CO2 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 CO2 percentage from exceeding about 1 per cent, 

 where no other gaseous impurities than CO2 are to 

 be found. 



One of the most interesting examples of the effects 

 of CO2 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 CO2 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 



