VI. In what way rarefaction takes place.
But with regard to rarefaction and condensation, whoever gives his
attention to his own thoughts, and admits nothing of which he is not
clearly conscious, will not suppose that there is anything in those
processes further than a change of figure in the body rarefied or
condensed: so that, in other words, rare bodies are those between
the parts of which there are numerous distances filled with other
bodies; and dense bodies, on the other hand, those whose parts
approaching each other, either diminish these distances or take them
wholly away, in the latter of which cases the body is rendered
absolutely dense. The body, however, when condensed, has not,
therefore, less extension than when the parts embrace a greater
space, owing to their removal from each other, and their dispersion
into branches. For we ought not to attribute to it the extension of
the pores or distances which its parts do not occupy when it is
rarefied, but to the other bodies that fill these interstices; just
as when we see a sponge full of water or any other liquid, we do not
suppose that each part of the sponge has on this account greater
extension than when compressed and dry, but only that its pores are
wider, and therefore that the body is diffused over a larger space.
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