D. More Probable Solution

50. Briefly, then, as to the first article, it seems more probable that, just as fire can exist outside its sphere in foreign matter, namely in an ignited body, as burning coal or flame (not that the form of fire is truly in the solid parts, unless it be posited that disparate specific parts together perfect the same matter, which seems unacceptable), so can the vapors existing in the air be ignited by juxtaposition [sc. with the sphere of fire]. And this successive ignition, now of these vapors, now of those (at least for all the air placed above the habitable region of men), can be called the conflagration.

51. And by it is the air well purified, because ignited bodies are converted at once into true and pure air. Since the air is predominant in its region, and since the ignited body, because of mutual contrary qualities in it, namely fire and vapor, resists the air a little (for it also in a way acts for the destruction of itself, but non-ignited vapor was not thus at once convertible by fire into pure air), it is apparent how flame thus has power for purifying gross air. For by the preceding action of an ignited body, and a body having a fiery quality in its watery self and having substantially the quality of water, the gross air is disposed by the containing body so as to be at once converted into what contains it.

And thus is pure air generated, which was not able thus to convert into itself a larger amount of gross vapor.According to the traditional theory of the disposition of the four elements in their pure state, earth is naturally below water which is naturally below air which is naturally below fire. Impure air is impure by containing earthly elements, but when earthly elements (body) are burning, they take on the quality of water (dissipating and becoming fissile, even exuding water), and so rise from the earthly to the watery element, that is, to the element next to the element of air. The previously impure air is now without most of its grosser elements and also now next to the element of air, by contact with which it becomes at once pure air. Thus can fire purify air of its grosser elements so that it becomes pure air. ³⁵