Thus the air passes through the middle of an obstacle by which it is 1 apparently divided. It not merely surrounds and encircles all objects, but permeates them likewise. It is shed abroad from the bright ether on high down to the very earth. It is nimbler and rarer and more exalted than the earth, and no less so than the waters of earth; but, on the contrary, it is thicker and heavier than the ether, and is naturally cold and dark, its light and heat coming from without. It is not of the same specific quality in every region, but borrows its qualities from its surroundings. The highest part of it is extremely dry and 2 hot, and so, very rare also, from the proximity of the eternal fires, the endless motions of the stars, and the constant revolution of the heavens. But the lowest portion next the earth is dense and dark, because it forms a receptacle for the exhalations of the earth. The intermediate portion, in dryness and rarity, runs to neither extreme as compared with the highest and lowest strata, but is colder than either. The reason is this: The higher parts are affected by the heat of the 3 heavenly bodies that are close by; and again the lower parts are warmed in the first place by the earth’s breath which is charged with heat, while in addition the sun’s rays are reflected from the ground, and as far as the reflection extends it renders the atmosphere kindlier and more genial. Besides, the temperature of the lower air is raised by the warm breath of all animals, trees, and crops, whose life is dependent on heat. Add to this also fires on the earth, not merely the 4 artificial ones about which we know, but also those concealed beneath it, some of which have ere this broken out, and myriads of which are blazing away in the hidden depths incessantly. Add, too, that all the fertile parts of the earth have some degree of heat which is exhaled into the air: heat is a condition of generation, the frigid is sterile. So, then, the middle portion of the atmosphere being remote from all these influences abides in its native cold: for air is by nature chilly. XI