Atmospheric air consists of
| Oxygen | 8 | Parts by weight. |
| Nitrogen | 28 |
Whence, for the combustion of one pound of carbon, we require
| Carbon | 1.00 |
| Oxygen | 2.66 |
But to obtain 2.66 of oxygen from the atmospheric air, we also use nitrogen in the proportion of 28 nitrogen to 8 oxygen; whence, for converting one pound of carbon to carbonic acid, we require
| Oxygen | 2.66 | |
| Nitrogen | 9.31 | |
| Or | 11.97 | lbs. of atmospheric air. |
From careful observations on the gases passing through the chimneys of well-constructed boilers, oxygen is found free, varying in amount from one quarter to one half of the quantity necessary for combustion; this is owing to the mechanical obstructions to the perfect conversion of the air arising from leakage through the fuel.
More than the above 11.97 lbs. of air should, therefore, be applied to the fire for each pound of carbon consumed. Twenty-five per cent. is found by experience to be a sufficient surplus allowance to convert the carbon.
| Whence, to | 11.97 | |
| add | 3.03 | |
| and we have | 15.00 | lbs. of atmospheric air per lb. of carbon. |
Air weighs .075 lbs. per cubic foot, whence 15
.075 or 200 cubic feet of air are necessary for the proper combustion of one lb. of pure carbon.