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On The Heavens   



reason is that there is more void. And this would also account for the

fact that a body composed of a number of solid parts equal to, or even

smaller than, that of another is sometimes larger in bulk than it.

In short, generally and in every case a body is relatively light

when it contains a relatively large amount of void. This is the way

they put it themselves, but their account requires an addition.

Relative lightness must depend not only on an excess of void, but also

an a defect of solid: for if the ratio of solid to void exceeds a

certain proportion, the relative lightness will disappear. Thus

fire, they say, is the lightest of things just for this reason that it

has the most void. But it would follow that a large mass of gold, as

containing more void than a small mass of fire, is lighter than it,

unless it also contains many times as much solid. The addition is

therefore necessary.

Of those who deny the existence of a void some, like Anaxagoras

and Empedocles, have not tried to analyse the notions of light and

heavy at all; and those who, while still denying the existence of a

void, have attempted this, have failed to explain why there are bodies

which are absolutely heavy and light, or in other words why some

move upward and others downward. The fact, again, that the body of

greater bulk is sometimes lighter than smaller bodies is one which

they have passed over in silence, and what they have said gives no

obvious suggestion for reconciling their views with the observed

facts.

But those who attribute the lightness of fire to its containing so

much void are necessarily involved in practically the same

difficulties. For though fire be supposed to contain less solid than

any other body, as well as more void, yet there will be a certain

quantum of fire in which the amount of solid or plenum is in excess of

the solids contained in some small quantity of earth. They may reply

that there is an excess of void also. But the question is, how will

they discriminate the absolutely heavy? Presumably, either by its

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