Plutarch Quaestiones Convivales 8.3.2

[2] Boethus began, and said: When I was a novice in letters, I then made use of geometrical postulates, and assumed as undoubted truths some undemonstrated suppositions; and now I shall make use of some propositions which Epicurus hath demonstrated already. Bodies move in a vacuum, and there are a great many spaces interspersed among the atoms of the air. Now when the air being rarefied is more extended, so as to fill the empty space, there are but few vacuities scattered and interspersed among the particles of matter; but when the atoms of air are condensed and laid close together, they leave a vast empty space, convenient and sufficient for other bodies to pass through. Now the coldness of the night makes such a constipation. Heat opens and separates the parts of condensed bodies. Therefore bodies that boil, grow soft, or melt, require a greater space than before; but on the contrary, the parts of the body that are condensed or freeze are contracted closer to one another, and leave those vessels and places from which they retired partly empty. Now the voice, meeting and striking against a great many bodies in its way, is either altogether lost or scattered, and very much and very frequently hindered in its passage; but when it hath a plain and smooth way through an empty space, and comes to the ear uninterrupted, the passage is so sudden, that it preserves its articulate distinctness, as well as the words it carries. You may observe that empty vessels, when knocked, answer presently, send out a noise to a great distance, and oftentimes the sound whirled round in the hollow breaks out with a considerable force; whilst a vessel that is filled either with a liquid or a solid body will not answer to a stroke, because the sound hath no room or passage to come through. And among solid bodies themselves, gold and stone, because they want pores, can hardly be made to sound; and when a noise is made by a stroke upon them, it is very flat, and presently lost. But brass is sounding, it being a porous, rare, and light metal, not consisting of parts closely compacted, but being mixed with a yielding and uncompacted substance, which gives free passage to other motions, and kindly receiving the sound sends it forward; till some touching the instrument do, as it were, seize on it in the way, and stop the hollow; for then, by reason of the hindering force, it stops and goes no farther. And this, in my opinion, is the reason why the night is more sonorous, and the day less; since in the day, the heat rarefying the air makes the empty spaces between the particles to be very little. But, pray, let none argue against the suppositions I first assumed.