294, and we surely see them: and also certain things lighter than fire could exist, like certain vapors, which rise above fire; but we cannot confidently affirm this because we have not been above fire.But even if it is fire, it is however not deprived of all heaviness; for that belongs to the void: that is why fire also, if air is removed from under it, will go down, if void or any other medium less heavy than fire is left under it.For all things go down, provided that they are heavier than the medium through which they must be carried, as has been shown above {1}; and there is nothing against motion taking place in the void, as was also made clear {2}.
But in fact fire does not go down because the air, through which it would have to be carried, is heavier than fire itself, and not because fire has no heaviness: just as air does not go down, because it would have to be carried through water, which, since it is heavier than air, does not permit this; and it must not be said that air is deprived of all heaviness because it does not go down.
Chapter [13] In {1} which it is demonstrated against Aristotle and Themistius, that it is only in the void that the differences of heavinesses and motions can be discerned with exactness.
Themistius, following Aristotle's opinion, in discussing the void, on text #74 of Book IV of the Physics [216a9-21] has written this: Since, then, the void yields uniformly, but as a matter of fact it yields in no way (for since it is nothing, the man who can imagine the void yielding is clever indeed), it so happens that the differences between heavy things and light things, that is the variations of things, are suppressed, and, in consequence, the speed of all things that are moved comes to be equal and indiscriminate. Now how false these words are will soon be known, when we have made clear how it is only in the void that the true differences between heavinesses and motions can be given, and how in a plenum these can in no way be found.
And, to begin with, just as among philosophers different opinions on the same subject indicate with sure evidence that none of them has discovered the truth (for if it had once been found by someone, immediately and without controversy, being what it is by its nature, it would have allowed itself to be seen and known by all), so, in the same way, the different ratios of the heavinesses of the same bodies in different media prove with a strong argument, that the true natural weights are not determined by any medium. For the heavier a medium is, the greater is the difference between the heavinesses of solids.In order that this may be understood still more easily, let those things