Artificial heat for household purposes is obtained by rapid combustion, or the chemical union of the oxygen in the air with the carbon and hydrogen found in fuel.

Wood, charcoal, anthracite and bituminous coals, kerosene oil, and gas, all are composed of either carbon, or compounds of carbon with a gas, hydrogen, forming hydro-carbons. Sometimes they contain both.

All these varieties of fuel were originally derived from vegetable matter. The living tree or plant, through its leaves and roots, takes in from the air and soil carbonic acid gas and water with earthy and nitrogenous matter dissolved in the water. It gives back to the air a large part of the oxygen contained in the gas, but retains some of it, and especially retains much of the carbon and water. Upon these it lives, and from these, with the help of the sunlight, it constructs the woody fibre, sap, and other substances, - compounds which are rich in carbon. Since these compounds have been built up by the energy of the sunlight, and can unite with oxygen, they are readily combustible. When we burn them in the form of wood, oil, fat, etc., this energy is liberated, or set free, as heat, or light, or both. By heat, which represents a certain amount of energy, we are enabled to have work done: on a large scale, when we burn coal under an engine, and on a small scale when we burn it in our stoves, and use the heat to cook our food. Artificial heat may thus be traced to the sunlight, the chief source, also, of all natural heat.