The disengagement of heat and light which accompanies chemical combination. Combustion was for a long time held to be the disengagement or development" of a certain air supposed to be contained in all combustible bodies, and to which the name of phlogiston was assigned; and bodies which had undergone combustion were said to be dephlogisticated, or to have parted with their phlogiston. This hypothesis, notwithstanding the numerous facts at variance with it, and particularly the familiar one that metallic oxides formed by combustion increase in weight, which could not be the case if combustion consisted in depriving them of any substance previously contained in them, was zealously maintained by many philosophers long after the brilliant discoveries of Scheele, Cavendish, and Priestley, as to the nature of atmospheric air, and against the theory of Lavoisier, founded upon these discoveries, and upon Dr. Black's theory of latent heat. According to the theory of Lavoisier, the light and heat emanate from the oxygenous portion of the atmosphere at the moment of its fixation with inflammable bodies; and succeeding chemists of his school, enlarging upon this idea, have ranged all substances under two classes, viz. combustibles, and supporters of combustion.

But this theory, although much more consistent with most of the phenomena of combustion, is still insufficient to account for some of them, and the distinction between combustibles and supporters of combustion appears to be altogether imaginary, for one substance is frequently in both capacities, being at one time apparently a supporter, and at another time a combustible. Thus, sulphuretted hydrogen is a combustible with oxygen, and chlorine a supporter with potassium; and sulphur, with chlorine and oxygen, acts as a combustible, but with the metals it becomes a supporter. Nor can we ascribe the appearances to an extrusion of latent heat in consequence of condensation: the protoxide of chlorine, a body destitute of any combustible constituent at the instant of decomposition, evolves light and heat with explosive violence, and its volume becomes one-fifth greater; and the chlorates and nitrates in like manner treated with charcoal, sulphur, or metals, detonate or deflagrate, while the volume of the combining substances is greatly enlarged. Neither can the heat evolved in combustion be attributed to a diminished capacity for heat in the resulting substances, for in the greatest number of cases there is no diminution of capacity of the compounds formed.

For example, the combination of oxygen and hydrogen, or of sulphur and lead, produces no greater alteration in the capacity of water, or of the sulphuret of lead, than the combination of oxygen with copper, lead, or silver, or of sulphur with carbon, produces in the capacities of the oxides of these metals, or of carburet of sulphur. The preceding, and numerous similar facts, prove that combustion is not necessarily dependent upon the agency of oxygen; that the evolution of the heat is not to be ascribed simply to a gas parting with its latent store of that ethereal fluid on its fixation or combustion; and that no peculiar substance, or form of matter, is necessary for producing the effect, but that it is a general result of the actions of any substances, possessed of strong chemical attractions, or of different electrical relations; and that it takes place in all cases in which a violent motion can be conceived to be communicated to the corpuscles of bodies. For this view of the subject the world is indebted to the masterly mind and deep and successful researches of Sir H. Davy. For a clear and comprehensive account of his experiments connected with this subject, together with his inferences therefrom, we refer the reader to Dr. Ure's Chemical Dictionary.