Diogenes Laertius, Lives of the Eminent Philosophers.
Two passages on propagation: Stoic sound as spreading waves (VII.158) and
Epicurean atomic motion below the threshold of sense (X.61-62).

Text: English Wikisource, R. D. Hicks translation (1925).
  Book VII: https://en.wikisource.org/wiki/Lives_of_the_Eminent_Philosophers/Book_VII
  Book X:   https://en.wikisource.org/wiki/Lives_of_the_Eminent_Philosophers/Book_X
Rights: Hicks's 1925 translation; Wikisource text. Footnotes and Greek
glosses removed; section numbers kept in square brackets.
Retrieved and verified 2026-09-18.

Date: Diogenes Laertius compiled in the 3rd century CE, reporting Stoic
doctrine of the 3rd century BCE onward and quoting Epicurus (341-270 BCE)
verbatim in Book X. The Epicurus letter is a primary document inside a
later compilation.

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VII.158  Stoic hearing: the air struck spreads in spherical waves
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  so Chrysippus in the second book of his Physics and Apollodorus. The apex of the cone in the air is at the eye, the base at the object seen. Thus the thing seen is reported to us by the medium of the air stretching out towards it, as if by a stick. [158]. We hear when the air between the sonant body and the organ of hearing suffers concussion, a vibration which spreads spherically and then forms waves and strikes upon the ears, just as the water in a reservoir forms wavy circles when a stone is thrown into it. Sleep is caused, they say, by the slackening of the tension in our senses, which affects the ruling part of the soul. They consider that the passions are caused by the variations of the vital breath. Semen is by them defined as that which is capable of generating offspring like the parent. And the human semen which is emitted by a human parent in a moist vehicle is mingled with parts of the soul, blended in the same ratio in which they are present in the parent. {{

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X.61-62  Epicurus, Letter to Herodotus: equal speed, and the continuity
         of motion that sense reports but reason denies
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  anchor+|61}}. “When they are travelling through the void and meet with no resistance, the atoms must move with equal speed. Neither will heavy atoms travel more quickly than small and light ones, so long as nothing meets them, nor will small atoms travel more quickly than large ones, provided they always find a passage suitable to their size, and provided also that they meet with no obstruction. Nor will their upward or their lateral motion, which is due to collisions, nor again their downward motion, due to weight, affect their velocity. As long as either motion obtains, it must continue, quick as the speed of thought, provided there is no obstruction, whether due to external collision or to the atoms’ own weight counteracting the force of the blow. [62]. “Moreover, when we come to deal with composite bodies, one of them will travel faster than another, although their atoms have equal speed. This is because the atoms in the aggregates are travelling in one direction during the shortest continuous time, albeit they move in different directions in times so short as to be appreciable only by the reason, but frequently collide until the continuity of their motion is appreciated by sense. For the assumption that beyond the range of direct observation even the minute times conceivable by reason will present continuity of motion is not true in the case before us. Our canon is that direct observation by sense and direct apprehension by the mind are alone invariably true. {{

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