DISCOURSE IV.

ON THE NATURE AND PROPERTIES

OF THE AETHER.

Although the ancient philosophers, especially the Aristotelians, held the Aether to be a "fifth essence" original: "quinta essentia"—incorruptible and different in its entire kind from the other elements—more recent thinkers, especially since Descartes, have begun to consider it a most subtle matter, differing from other elements not in species, but only in its subtlety.

Hence arises the question: Is the Aether a simple body or a compound one? And is it fluid or solid?

To the first question we answer: The Aether is a most simple body, consisting of the smallest spherical particles in continuous motion. This is evident because light, which propagates through the Aether, moves in a straight line; this would not happen if the Aether were a heterogeneous body consisting of particles of different shapes.

To the second question: The Aether is fluid, and indeed highly fluid. For if it were solid, celestial bodies like the planets could not move through it except with great resistance, which is refuted by experience. Planets move through celestial spaces without any sensible retardation, which demonstrates that the medium through which they move is highly fluid and offers almost no resistance.

Descartes on the Aether.

Descartes, however, considers the Aether to be a subtle matter generated from the friction of solid bodies, especially in circular motion. This matter, he says, is what fills the heavens and transmits light from the sun to us. But this opinion labors under many difficulties, which it is not our purpose to pursue here.