The heat-producing elements, i. e., the combustible substances, of all fuels, whether solid, liquid, or gaseous, are carbon and hydrogen. Many fuels, particularly coal, contain free sulphur, which is always more or less objectionable, sometimes seriously so, and although sulphur produces heat when burned, its quantity is too small to be considered in summing up the heat value of a fuel.

Almost all fuels contain ash, water and other matter which will not burn, and these determine largely the practical and commercial values of all fuels. These are of negative value in fuel, as they absorb and carry away heat when the fuel is burned.

The heating or calorific value of a fuel is usually expressed in British Thermal Units. One B. T. U. is that quantity of heat necessary to raise the temperature of one pound of pure water through 1° F., at or near 39.1° F., its point of greatest density. The heat value of a fuel is stated as that quantity of B. T. XL's which one pound of the fuel will evolve when burned.

In commercial practice, fuel is usually sold by weight, but the heat value of a fuel is only crudely expressed by its weight. The value of a fuel for practical purposes is far better expressed by the number of heat units which a given weight of it will supply when burned. The desirability of coal is also determined by the sulphur it contains, and by the character and quantity of the ash it makes, as some coals produce a sticky clinker hard to remove from the grate ' bars.