ordinary sense, Fire is understood to mean matter in a state of combustion or incandescence: and it is in this acceptation more particularly, that the term is used in the present work. During the eighteenth century, the supposed general principle of heat, or inflammability, was called phlogistion, a term invented to suit the theory of Stahl, which assumed and thus designated such principle, as a constituent element of all combustibles. But Lavoisier, who died in 1794, introduced a new theory, depending on the existence of what is called caloric, a denomination universally adopted by modern chemists. It assumes that combustion is caused by the combination of the oxygen of the atmosphere, not with hydrogen, or with the imaginary substance of phlogistion, but with the combustible itself, and that in such combination light and heat are produced.* It would be out of place in a work like the present, to enter into any lengthened investigation of the nature of this agency to which the phenomena of light and heat are ascribed; or in what respects it seems to fail to account for some existing facts. It may be remarked, however, that by one class of theorists, heat has been hypothetically regarded as a fluid of inappreciable tenuity, whose particles are endowed with indefinite ideo-repulsive powers, and which, by their distribution in various proportions among the particles of ponderable matter, modify cohesive attraction, giving birth to the three general forms of gaseous, liquid, and solid.† Another class of philosophers, among whom ranks the celebrated Sir Humphrey Davy, have doubted the separate entity of a calorific matter, and have adduced
*Lardner's Treatise on Heat, p. 365.