In this division of his work the author discusses various aspects of the atmosphere and offers an explanation of the phenomena which he describes. He distinguishes between the very bright ether on high, and the moist, denser atmosphere which underlies it, but thinks that they must pass insensibly into each other (66). The atmosphere he regards as a continuous non-composite body, capable of great range in tension, and forming the vehicle through which the exhalations from the earth pass outwards to the sky. It does not everywhere possess the same qualities. In its lower parts next the earth it is dense and misty, owing to the terrestrial exhalations, and is there warmed by the earth’s breath, by the reflection of the sun’s rays from the ground, and from the fires, artificial and subterranean, as well as from the warmth communicated by living animals and plants, for life cannot exist without heat. The highest portions of the atmosphere are exceedingly dry, hot, and attenuated, owing to their nearness to the eternal fires and the heat of the heavenly bodies. The middle parts, on the contrary, are intermediate in character, but colder than what lies above and below them (60, 61). It is the lower portions that are subject to the greatest changes, for they receive the earthly elements which involve such constant turmoil. The instability of the air arises also in part from the motions of the earth and from those of the sun, moon, and stars, to which cold, rain, and other atmospheric disturbances are due (56, 61).