The Philosophy of Roger Bacon

Air

...that fire ought to [be...] as is evident from the earth and the water of the sea, which are objects of sight toward which a sharp sight is directed. Again, which [must be] worked through the medium of the earth. It is proved that through itself it encounters the dense original: dencium, likely referring to densum for which it was created; and thus however [it is said?]
Air

the back should not become a force indicating all those [things] that I myself and we are, because that which is rare rarum: a substance of low density, the opposite of dense cannot be [touched] by the hand. Whence it is said [that] all [leads] to the true transparent original: perspicuum; a medium through which light passes, like air or water as authors clearly teach through
Fire

earth and water; for they exist on account of another globe meaning a celestial sphere and of plants and of the divine powers. Thus from earth and water, for [from them] all things are born to withstand; but air is truly [shining?] for other breathing things; since in it
they cannot [be reached] by sight. But just as it can [be...] for they are types. Furthermore, because that which is in all its own species and in its genus, a coming [author?] is divined, although it is evident through itself through its brevity and altitude.

Hot Fire

For just as in this way the combination is [read?] in the earth, let it be called cold and dry. Water is cold and moist, air is pure and hot, Fire is hot and dry.
Because if one of these [is limited?], the properties exist and to the dry original: siccum it is absorbed, because they can be generated from one another.
It will be as he says concerning the light leve: "levity" or lightness, the natural tendency of an element to rise and that which is natured [to be] strongly under all bodies in common. And that in no place is there [pure] levity.
Whether it is in the air, that which is natured to a [certain] degree is light. Likewise, toward the place of the world—that is, in this world—it is absurd to look through the light
[for] it is that which is natured so that all things moved are moved by straight motion motu recto: motion in a straight line, which Aristotle attributed to the four changing elements of the earthly realm and then [it is] curved, which is not [so], or rather the long light [thing] and then
that which is not here but is in such a place heavy grave: "gravity" or heaviness, the natural tendency to sink toward the center of the universe. Here are the points which [form the] axis through the heaven... through the world. It can also be confirmed by authority. If we, seeing that the whole world is not heavy—that which is water or
earth—or not only is there levity in the air, which is water.
But light [before?] and here note that fire and water are not heavy in their own place, nor in the place of air and fire, which in these elemental places are natured to descend.
If earth is [displaced], but in the place of earth it is light, for if it were in the place of earth, it would only become [heavy] as far as the gravity of air.
Just as air is and was below the sphere, and it is not supremely light because only all that which exists in the truly light air. [By its] mass, by straight motion, and above
it again; and here is fire, however, the sphere [exists]. And in all [the stars?] are moved by straight motion which is [contrasted with] mass in circular motion motu circulari: the eternal, perfect motion of the celestial bodies, unlike the straight-line motion of earthly elements.
Likewise, since it is necessary that what is supremely light becomes supremely heavy.