Sentence:  "Combustibility", then, is not a quality of the combustible, "taken by itself". It is merely a faculty which may be brought into action through the instrumentality of a corresponding faculty in some other body. It is, in the case now before us, the union of the "combustible" with "oxygen", and which, for this reason, is called the "<WSD>"supporter</WSD>""; neither of which, however, "when taken alone", can be consumed. To effect combustion, then, we must have a "combustible" and a <WSD>"supporter</WSD>" of combustion. 
Meaning of the word marked with <WSD> tags:  Something that supports another thing.