ON THE NATURE AND PROPERTIES

OF THE AETHER.

Since we have dealt in the preceding discourse with the sublunary elements—their qualities and movements—order now demands that we turn our attention to the upper part of the world, namely, the Aether, or the Heavens. The Aether, as defined by philosophers, is a luminous, subtle, and incorruptible body that constitutes all celestial bodies.

Definition of the Aether.

However, a distinction must be made between the Aether, which Aristotle calls the "fifth essence," and the Aether which more recent physicists consider to be a certain most subtle fluid that penetrates all bodies. We shall discuss here the nature of the Aether as it relates to the movements of celestial bodies and the propagation of light.

Properties of the Aether.

The principal properties of the Aether are:
1. Extreme subtlety, by which it fills all interplanetary spaces.
2. Luminosity, or the capacity to transmit light.
3. Incorruptibility, by which it is immune to the mutations of sublunary elements.

On the movement of the Aether.

Regarding the movement of the Aether, the ancients held it to be circular, as befitting celestial bodies. More modern thinkers, however, who do not admit a vacuum in nature, consider the Aether to be a medium through which light rays and forces of attraction are propagated.