IX
To the foregoing Democritus’ explanation falls to be added. Every body receives heat more quickly and retains it longer in proportion to its solidity. For example, if three vessels, of copper, glass, and silver respectively, are set in the sun, the heat will penetrate the copper one soonest and will remain in it longest. The reason why Democritus is of this opinion may also be added. In the bodies, he says, that are harder, more compact, and dense, the openings must of necessity be smaller than in others, and in each of the openings the film of air must be thinner. It follows that just as smaller baths and smaller cylinders are heated more rapidly than others, so these concealed apertures, so small as to elude the eye, both feel the heat more quickly, and by reason of this same smallness of calibre give back more slowly the heat they have received.