Combustion is the term commonly applied to those chemical processes which are accompanied in a marked degree by the production of heat and light. The most familiar of such processes are those in which oxygen of the atmosphere combines chemically with the constituents of what are ordinarily spoken of as combustible substances, such as wood, coal, fats, oils, &c. Chemical combination is, as a rule, accompanied by the evolution of heat and frequently of light (see CHEMISTRY); but every case of chemical combination is not called combustion, because in many cases the quantity of heat evolved is inconsiderable.
When we speak of the combustion of, for instance, coal or wood, we mean the chemical process which consists, in general terms, in the combination of the oxygen of the air with the carbon and hydrogen which constitute the greater part of the combustible portion of either of these substances, and in the production of carbonic acid and water. Many instances are known to chemists in which the oxygen required for a combustion is not derived directly from the atmosphere, but from some oxidising (or oxygen-yielding) agent, and frequently the place of the oxygen may be altogether taken by some other element, as, for instance, when metals, such as antimony in powder, burn in chlorine.
The combustion of every combustible substance is accompanied by the evolution of a quite definite quantity of heat, which is invariable for each substance, whether the combustion takes place rapidly or slowly. A piece of phosphorus, for instance, as is well known, glows in the dark. This is a process of very slow combustion, and is never accompanied by much rise of temperature. When moderately heated in air, a piece of phosphorus bursts into flame, and the combustion is rapid and is accompanied by a considerable rise of temperature; whilst if the phosphorus be burned in pure oxygen, the combustion is an extremely brilliant spectacle, and a high temperature is attained. In each case the actual quantity of heat given out is identical for the same weight of phosphorus, but the time occupied by the combustion varies, and consequently the temperature at any given instant must also vary.
The quantities of heat given out by the combustion of the same weights of different substances vary greatly. The measurement of quantities of heat produced by combustion and in other ways is called calorimetry (see HEAT).
The name combustion is applied to a particular process in the analysis of organic compounds. See ANALYSIS (ORGANIC). See also SPONTANEOUS COMBUSTION.