Chapter Xi. of Heaven, What Is Its Nature and Essence.

Anaximenes affirms that the circumference of heaven makes the limit of the earth's revolution. Empedocles, that the heaven is a solid substance, and hath the form and hardness of crystal, it being composed of the air compacted by fire, and that in both hemispheres it invests the elements of air and fire. Aristotle, that it is formed by the fifth body, and by the mixture of extreme heat and cold.