ON THE NATURE AND PROPERTIES

OF THE AETHER.

Since we have dealt in the preceding discourse with the sublunary elements, their qualities, and their movements, as far as seemed sufficient for our purpose, it remains now to discuss a few things concerning the Aether, or the Celestial Element. For the Aether, as defined by philosophers, is a simple, transparent, and incorruptible body that fills the heavens and contains the stars.

Aristotle, On the Heavens, book 1, chapter 3.

Aristotle, in his book On the Heavens, distinguishes the Aether from the remaining elements, asserting that it is neither heavy nor light, but a certain fifth nature, distinct from the four elements. This nature, as he says, is perpetual and immutable, and suffers no mutation or alteration.

On the properties of the Aether.

The principal properties of the Aether are:
1. Simplicity: Because it does not consist of heterogeneous parts, but is a pure and uniform body.
2. Transparency: Because it transmits light through itself and is translucent.
3. Incorruptibility: Because it is not subject to generation or corruption, as are the sublunary elements.
4. Circular motion: Because it is naturally moved by circular motion, which is most perfect and eternal.

Is the Aether fluid?

It is asked here whether the Aether is fluid or solid. Many of the ancient philosophers thought it was solid, because of the perceived solidity of the celestial spheres. But more recent astronomers, considering the movement of the planets, contend that it is fluid and behaves like water or the most subtle air, through which the stars are carried as if in a fluid.