acquainted with several of the properties of the air, but,
as they had not been tested experimentally, its acquaint-
ance with them was but conjectural, and could lead to no
positive inference; it was reserved for modern science to
ascertain what the air is, and what its properties in rela-
tion to the world, its productions and inhabitants.
Note 3, p. 101. A void is rightly said, &c.] It would be difficult even to conjecture what could have been meant by a void in that age ; for although it had been perceived[1], it may be but obscurely, that the air rises by fire (heat) to the upper regions and becomes ether, (as in the Timæus, expiration is accounted for by the rising up from within of the heated breath,) yet it is not to be supposed that rarefaction was an admitted property of the air, or that any condition like rarefaction was implied in the void. Aristotle[2] observes, upon this topic, that, "according to some philosophers, a plenum is a space or vessel when full, and a vacuum or void is the same when empty, thus making, as he says, the plenum to be identical with the vacuum and space, excepting in conditions of relation." In all this it is evident that no account was taken of the air; and he objects to Anaxagoras, (who had shewn, experimentally, that the air is substance of some kind,) that he argues against what had never been contended for—the advocates for a void maintain, he says, that it is a space in which there is no tangible body, and, holding every
