Corpus Hermeticum with Pneumatica and Ocellus Lucanus

...providing for the most necessary requirements of life, while others display a certain astounding wonder.

† Concerning the Vacuum †

Before discussing what follows, we must first treat the subject of the vacuum original: "κενοῦ" (kenou). In ancient physics, the existence of a "void" or empty space was a major point of debate between different schools of philosophy.. For some say there is absolutely no vacuum; others say that while no continuous? vacuum exists according to nature, there is one scattered in small particles through air, water, and other bodies. It is to these latter thinkers that we should most closely agree. For from the phenomena and from things falling under our direct perception, it will be shown in the following pages that this is what occurs.

For if vessels that seem empty to most people are not, as they suppose, actually empty, but are full of air—and air is a body original: "σῶμα" (sōma). In this context, "body" means a physical substance or matter that occupies space., as is agreed upon by those who study natural philosophy—it is composed of small and fine-particled bodies, which are for the most part invisible to us. If, therefore, one pours water into a vessel that appears to be empty, as much volume of water as enters the vessel, a corresponding volume of air will be displaced.

One might understand what is being said from the following experiment. For if one turns a vessel that seems to be empty upside down and [plunges it] into water... Heron is describing a classic experiment to prove air is matter: an inverted cup pushed into water will not fill with water because the air trapped inside acts as a physical barrier.