ON THE NATURE AND PROPERTIES

OF THE AETHER.

Since we have dealt in the preceding discourse with the sublunary elements and their affections, as far as seemed sufficient for our purpose, it remains now to discuss a few things concerning the Aether, or the Celestial body. We define the Aether not as Aristotle did—as a "fifth essence" different in kind from the remaining elements—but rather as modern physicists do: a fluid, subtle, and translucent body in which the stars swim as if in the open sea.

Definition of the Aether.

The nature of this Aether, although not perceived immediately by our senses, is sufficiently known from the effects and movements of celestial bodies. It is, as we have said, an extremely subtle body, subject to no resistance, and most apt for local movement—whether circular or straight, should necessity require it.

Properties of the Aether.

As for its properties, the principal ones are its invariability and incorruptibility. This is not to say that the Aether is devoid of all change (for we see new stars arise and vanish in it from time to time), but that it is immune to the mutations to which our own elements are subject. It is not generated, nor corrupted, nor rarefied, nor condensed like air or water; rather, it persists in the same state with perennial constancy.

Invariability.

In addition to this is its translucency, by which it diffuses the light received from the stars throughout the whole space of the world. If the Aether were opaque, or possessed any density, the light of the stars could not reach us, or would be greatly weakened. But since it is extremely transparent, it transmits light rays to us without any refraction or diminution.

Translucency.

Finally, the fluidity of the Aether is what permits the movement of planets and comets without the bother of any friction. If the Aether were solid or viscous, the movement of celestial bodies would be impeded, and the celestial spheres, if there were any, would collide with one another. But since it is most fluid and thin, celestial bodies move in it with the greatest freedom and complete their course without any impediment.

Fluidity.

These are the things which we deemed worthy of brief notation concerning the Aether and its nature. If anyone wishes to examine these things more deeply, let him consult the astronomical and telescopic observations which, in our times, have brought so much light to physicists.