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On The Heavens   

evidently in them the attributes are due to the difference of their

uncompounded parts: that is to say, according as the one or the

other happens to preponderate the bodies will be heavy and light

respectively. Therefore we need only speak of these parts, since

they are primary and all else consequential: and in so doing we

shall be following the advice which we gave to those whose attribute

heaviness to the presence of plenum and lightness to that of void.

It is due to the properties of the elementary bodies that a body which

is regarded as light in one place is regarded as heavy in another, and

vice versa. In air, for instance, a talent's weight of wood is heavier

than a mina of lead, but in water the wood is the lighter. The

reason is that all the elements except fire have weight and all but

earth lightness. Earth, then, and bodies in which earth preponderates,

must needs have weight everywhere, while water is heavy anywhere but

in earth, and air is heavy when not in water or earth. In its own

place each of these bodies has weight except fire, even air. Of this

we have evidence in the fact that a bladder when inflated weighs

more than when empty. A body, then, in which air preponderates over

earth and water, may well be lighter than something in water and yet

heavier than it in air, since such a body does not rise in air but

rises to the surface in water.

The following account will make it plain that there is an absolutely

light and an absolutely heavy body. And by absolutely light I mean one

which of its own nature always moves upward, by absolutely heavy one

which of its own nature always moves downward, if no obstacle is in

the way. There are, I say, these two kinds of body, and it is not

the case, as some maintain, that all bodies have weight. Different

views are in fact agreed that there is a heavy body, which moves

uniformly towards the centre. But is also similarly a light body.

For we see with our eyes, as we said before, that earthy things sink

to the bottom of all things and move towards the centre. But the

centre is a fixed point. If therefore there is some body which rises

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