Solid Fuels.—The great fuels in nature are carbon and hydrogen, carbon being the substance most widely known and depended upon. Hard coal, for instance, is composed almost wholly of carbon; whereas soft coal has a considerable quantity of hydrogen.
As coal was formed by wood, which, through long process of time became carbonized, it contains considerable foreign matter which will not burn, forming ash.
Liquid Fuels.—The volatile oils, however, have very little non-combustible matter. Ordinary petroleum contains about 80 per cent, of carbon, and from 12 to 15 per cent. of hydrogen, the residue being foreign matter, all more or less susceptible of being consumed at high temperatures.
Combustion.—The term combustion, in its general sense, means the act of burning; but in a larger and more correct application it refers to that change which takes place in matter when oxygen unites with it.
Oxygen is a wonderful element, and will unite with all known substances, unlike all other elements in this respect. It may take years for it to form a complete unity. Thus, wood, in time, will crumble, or rot, as it is called. This is a slow process of combustion, brought about without applying heat to it, the change taking place in a gradual way, because oxygen unites with only a small portion of the wood.
Oxidation.—Iron will rust. This is another instance of combustion, called oxidation. When oxygen unites with a substance it may produce an acid, or an alkali, or a neutral compound. When wood is burned it produces an ash, and this ash contains a large amount of potash, or lye, which is an alkali, or a salt. So when other substances are burnt the result may be an acid, like sulphur, or it may be unlike either acid or the alkali.
The unity of oxygen with the food in the body is another instance of oxidation, which produces and maintains the heat necessary for existence.
Carbon or hydrogen, as a fuel, are inert without oxygen, so that in considering the evolution of a force which is dependent on heat, we should know something of its nature, thereby enabling us to utilize it to the best advantage.
The Hydro-carbon Gases.—If petroleum, or gasoline, should be put into the form of a gas, and as such be confined in a receiver, without adding any oxygen, it would be impossible to ignite it.
The character of the material is such that it would instantaneously extinguish any flame. Now, to make a burning mixture, at least three parts of oxygen must be mixed with one of the hydro-carbon, before it is combustible.