COMBUSTION. The temperature of bodies may be raised by various means, which are generally such as produce an agitation among the particles. The sun's light, and also the chemical or mechani cal actions of bodies upon each other, if sufficiently intense or rapid, produce this effect. One of the most generally known methods of producing a high temperature consists in striking or rub. bing bodies together ; and there is no ac tion more familiar to us, for this pur pose, than the striking of a flint against a piece of steel. Whenever an elevated temperature is thus produced in a body communicating with the open air, it is observable that, according to the nature of the body itself, the heat is either con ducted away, and nothing farther hap pens, or else it continues, and even in creases, so as to spread by communica tion through every part of the body, and produce a change in BS nature. Thus, if one corner or extremity of a thin piece of stone or glass be made red hot, it will soon become cold again, and no farther effect will follow ; but if the corner of a piece of paper or wood be heated in like manner, it will not, in common circum stances, become cold again without al teration, but the beat will he communi cated to the whole mass, and will con tinue until the body shall have undergone a remarkable change. This phenomenon is called combustion or burning ; the bo dies which are liable to it are called com bustible; and after they have undergone this process they are said to have been burned.
There are scarcely any chemical changes, by which heat is produced, suffi cient to exhibit the appearance of light, unless oxygen be in the act of entering into combination with a combustible body. One of the earliest observations respect ing ordinary combustion must have been, that it cannot take place without com mon air, and that it is extinguished by shutting out the air. It is new well known, that the air acts only by means of its oxygen, which unites with and changes the combustible body.
The earlier doctrines respecting heat and fire are scarcely entitled to notice ; and certainly must not occupy our pages. It will he sufficient for us to remark, that the hypothesis of an element called fire, which was supposed to escape from burning bodies and ascend to a sphere a bove, was modified by Beecher and Stahli by the supposition of a general principle, assumed to exist in all combustible bodies, and denominated phlogiston ; capable of passing in combination from one body to another, or of flying off with a violent agitation, in which the heat was imagined to consist. As this theory was establish
ed upon the observation of a number of striking chemical facts, it was for a long time universally received. Various modi fications were, however, proposed by different chemists, as discoveries came to be made ; particularly with regard to the agency and combination of air in bo dies, and afterwards those of the exist ence of oxygen, and the laws by which heat, or the cause of temperature, is go verned. These advances led to the re jection of phlogiston altogether ; a change of theory, which was more rapid ly effected by the patronage, exertions, and scientific labour of Lavoisier ; who devoted the influence of an elevated situa tion, the extent of his fortune, and the powers of an uncommonly clear and comprehensive intellect, to this object. It is to be regretted that, with claims so well founded and so great, this philoso pher should have sought for more ; but it is certainly tme, that lie himself gave sup port to the powerful cry of that party, which has proclaimed him the author of the modern theory of combustion ; where as, if they had continued to do justice to Rey, Hooke, Mayow, Hales, Bayen, Priestley, and others, there would have been little of absolute facts left for La voisier to clai in in the way of original dis covery; though it would be difficult to find adequate terms to express the obli gation under which the scientific world is placed with regard to him, for his am ple and accurate repetition of experimen tal investigations, and the very luminous and able manner in which he has digest ed and stated the whole mass of facts, and applied them to theoretical results. Combustion, as understood by modern chemists, is the rapid combination of oxy gen with a body, which is attended with in crease of temperature and the emission of light. The burned body is therefore an oxygenated compound. Thus we may form a notion of combustion by burning a piece of iron wire. If the diameter of the wire be very small, such, for ex ample, as half the thickness of a hair, and it be made up into a tuft like wool, it may be lighted by a candle, and will burn, like other more readily combuSti tile bodies, until it has received a cer tain portion of oxygen, after which the combustion will cease. If the same iron had been exposed to the atmosphere with out additional heat, it would also have attracted oxygen, but in a longer time ; and though the result might have been the same, we should not have called this slow process by the name of com bustion.