II. Charcoal. The different charcoals afford, under equal weights, equal quantities of heat. We may reckon, upon an average, that a pound of dry charcoal is capable of heating 73 pounds of water from the freezing to the boiling point; but when it has been for some time exposed to the air, it contains at least 10 per cent. of water, which is partially decomposed in the combustion into carburetted hydrogen, which causes flame, whereas pure dry charcoal emits none.
A cubic foot of charcoal from soft wood weighs upon an average from 8 to 9 pounds, and from hard wood 12 to 13 pounds; and hence the latter are best adapted to maintain a high heat in a small compass. The radiating heat from charcoal fires constitutes one third of the whole emitted.
III. Pitcoal. The varieties of this coal are almost indefinite, and give out very various quantities of heat in their combustion. The carbon is the heat-giving constituent, and it amounts, in different coals, to from 75 to 95 per cent. One pound of good pitcoal will, upon an average, heat 60 pounds of water from the freezing to the boiling point. Small coal gives out three-fourths of the heat of the larger lumps. The radiating heat emitted by burning pitcoal is greater than that by charcoal.
IV. The coke of pitcoal.—The heating power of good coke is to that of pitcoal as 75 to 69. One pound of the former will heat 65 pounds of water from 32° to 212°; so that its power is equal to nine-tenths of that of wood charcoal.
V. Turf or peat.—One pound of this fuel will heat from 25 to 30 pounds of water from freezing to boiling. Its value depends upon its compactness and freedom from earthy particles; and its radiating power is to the whole heat it emits in burning, as 1 to 3.
VI. Carburetted hydrogen or coal gas.—One pound of this gas, equal to about 24 cubic feet, disengages in burning, as much heat as will raise 76 pounds of water from the freezing to the boiling temperature.
In the following table the fourth column contains the weight of atmospherical air, whose oxygen is required for the complete combustion of a pound of each particular substance.
| Species of combustible. | Pounds of water which a pound can heat from 0° to 212°. | Pounds of boiling water evaporated by 1 pound. | Weight of atmospheric air at 32°, to burn 1 pound. | ||
|---|---|---|---|---|---|
| Perfectly dry wood | 35·00 | 6·36 | 5·96 | ||
| Wood in its ordinary state | 26·00 | 4·72 | 4·47 | ||
| Wood charcoal | 73·00 | 13·27 | 11·46 | ||
| Pitcoal | 60·00 | 10·90 | 9·26 | ||
| Coke | 65·00 | 11·81 | 11·46 | ||
| Turf | 30·00 | 5·45 | 4·60 | ||
| Turf charcoal | 64·00 | 11·63 | 9·86 | ||
| Carburetted hydrogen gas | 76·00 | 13·81 | 14·58 | ||
| Oil | - | 78·00 | 14·18 | 15·00 | |
| Wax | |||||
| Tallow | |||||
| Alcohol of the shops | 52·60 | 9·56 | 11·60 | ||
The quantity of air stated in the fourth column, is the smallest possible required to burn the combustible, and is greatly less than would be necessary in practice, where much of the air never comes into contact with the burning body, and where it consequently never has its whole oxygen consumed. The heating power stated in the second column is also the maximum effect, and can seldom be realized with ordinary boilers. The draught of air usually carries off at least 1⁄7 of the heat, and more if its temperature be very high when it leaves the vessel. In this case it may amount to one half of the whole heat or more; without reckoning the loss by radiation and conduction, which however may be rendered very small by enclosing the fire and flues within proper non-conducting and non-radiating materials.