Fire, in Natural Philosophy, combustion, or the decomposition of combustible bodies, accompanied with light and heat. The word, however, has been used in such various senses by philosophers of different schools, that in works of close reasoning it is now generally exchanged for that of combustion, as a term affording a more definite meaning. Fire, under this view of the subject, is not a substance, but a quality. It supposes two or more bodies entering into combination, attended with an emission of light and heat. All these phenomena may take place separately, but it is a compound operation, resulting from the union of the whole, that alone produces fire.