IGNITION, properly signifies the setting fire to any substance; but the sense is commonly restrained to that kind of burning which is not accompanied with flame, such as that of charcoal, cinders, metals, stones, and other solid substances.
The effects of ignition are first to dissipate what is called the phlogiston of the ignited substance, after
which it is reduced to ashes. Vitrification next follows; and lastly, the substance is totally dissipated in vapour. All these effects, however, depend on the presence of the air; for in vacuo the phlogiston of any substance cannot be dissipated. Neither can a body which is totally destitute of phlogiston be ignited in such a manner as those which are not deprived of it: for as long as the phlogiston remains, the heat is kept up in the body by the action of the external air upon it; but when the phlogiston is totally gone, the air always destroys, instead of augmenting the heat. Philosophers have therefore been greatly embarrassed in explaining the phenomena of ignition. It hath been found a matter of no small difficulty to determine how far the air, and how far the phlogiston, is concerned in this operation; but as these inquiries cannot be entered into without first determining the nature of the PHLOGISTON, we must refer to that article for an account of the different theories of ignition.