"From this time oxygen began to be conventionally spoken of as the "supporter of combustion," and substances which burn in it were called "combustibles." Yet this manner of speaking is purely conventional, and is due to the abundance of oxygen in the atmosphere. Combustion is merely a special case, in which the heat of combination is exceptionally augmented. After the discovery of chlorine, bromine, and iodine, and when bodies were found to burn in the vapours of these substances, as well as in oxygen, the term "supporter of combustion" was extended to every substance capable of forming vapour in which others could burn; but as the same substance may act at one time as a "supporter," and at another time as a "combustible," the distinction has gradually become obsolete."