FUEL. This term is generally applied to combustibles used for the production of heat; also, less frequently, to combustibles such as oil, paraffine-oil (q.v.), used for lighting. Under articles COAL, COKE, etc., will be found details of the physical prop erties and chemical composition of the various fuels; the following observations bear chiefly on their economical application as sources of motive power.
The two elementary bodies to which we owe the heating powers of all our fuels, natural and artificial, are carbon and hydrogen. Coke, wood-charcoal, peat-charcoal, mid anthracite, contain little or none of the latter' element, and may be regarded as purely carbonaceous fuels. . But wood, peat, and most varieties of coal, contain hydro gen as well as carbon; and in their combustion, these two substances combine to pro duce volatile and combustible hydrocarbons, which are volatilized previous to being consumed, while a purely carbonaceous F. evolves no volatile matter until combustion has been effected.
These hydrocarbons are numerous and varied in composition (see CARBORYDROGENS); but when combustion is perfect, the amount of heat produced by any hydrocarbon is exactly what would have been produced had the hydrogen and carbon been burned separately. It will be of advantage, therefore, to study these two elementary combus
tibles iu succession, in order to estimate subsequently the combined effect where they conic together in the same fuel.
The heating power of a combustible, or the amount of heat generated by it, is usu ally expressed in degrees Fahrenheit on so many pounds' weight of water, But in estimating the temperature, or intensity of heat produced, we have to keep in view that different substances have different capacities for heat—that of water being generally assumed as unity. The number expressing this capacity is called the specific heat of the substance. Water 1000, carbonic acid 221, imply that while 1000 units of heat are reqUired to elevate the temperature of water any given number of degrees, only 221 units are required to elevate to the same temperature an equal weight of carbonic acid.