When therefore Job 37:18 says "that the heavens are strengthened with iron," "that the sky is strong as a molten mirror," his mind is not dealing with the material but with the Word of God, which can make a thing the softest by nature the hardest and the firmest. For what is softer than water? What is thinner or more subtile than air? Yet these things, which were the most subtile and the softest by nature, from being created by the Word preserve their form and motion with the greatest perfection and the greatest firmness. Whereas, had the heaven been formed of adamant or of any material infinitely harder still, it would by its rapid, long and continuous motion, have soon been broken in pieces or melted. In the same manner the sun, by his rapid motion, would melt in one day even if formed of the hardest material, were it not for the Word of God by which it was created. For motion is of itself very creative of heat. Hence Aristotle asserts that the lead of the arrow is sometimes melted by the velocity of its motion. These facts of nature therefore are miracles of God, in which the omnipotence of his Word is clearly discerned, exhibiting the wonder that the heaven, though softer and more subtile than water, and performing continuously the most rapid revolution, and that too with so vast a variety of bodies and their motions, should have existed and revolved so many thousands of years uninjured and unweakened! It was this that caused Job to say, "that the heavens were molten, as it were, of brass," Job 37:18; that they are by nature the softest of substances. How great the subtility of the air is in which we live, we ourselves know perfectly well; for it is not only not tangible, but not discernible. And the heaven, or ether, is still more subtile and thinner than the air or atmosphere. For its blue or sea-color or water-color appearance is not a proof of its density, but rather of its distance and its thinness; to which its rarified state, if you compare the thicker substances of the clouds, the latter will appear in comparison, like the smoke of wet wood when first ignited. It is to this extreme subtility, yet unaltered durability, that Job alludes as above mentioned. So philosophers have among them this celebrated maxim, "That which is humid is limited by no boundary of its own."