7 Therefore we read: “Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.” I have said what the word firmament in Scripture means. It is not in reality a firm and solid substance which has weight and resistance; this name would otherwise have better suited the earth. But, as the substance of superincumbent bodies is light, without consistency, and cannot be grasped by any one of our senses, it is in comparison with these pure and imperceptible substances that the firmament has received its name. Imagine a place fit to divide the moisture, sending it, if pure and filtered, into higher regions, and making it fall, if it is dense and earthy; to the end that by the gradual withdrawal of the moist particles the same temperature may be preserved from the beginning to the end. You do not believe in this prodigious quantity of water; but you do not take into account the prodigious quantity of heat, less considerable no doubt in bulk, but exceedingly powerful nevertheless, if you consider it as destructive of moisture. It attracts surrounding moisture, as the melon shows us, and consumes it as quickly when attracted, as the flame of the lamp draws to it the fuel supplied by the wick and burns it up. Who doubts that the æther is an ardent fire? If an impassable limit had not been assigned to it by the Creator, what would prevent it from setting on fire and consuming all that is near it, and absorbing all the moisture from existing things? The aerial waters which veil the heavens with vapours that are sent forth by rivers, fountains, marshes, lakes, and seas, prevent the æther from invading and burning up the universe. Thus we see even this sun, in the summer season, dry up in a moment a damp and marshy country, and make it perfectly arid. What has become of all the water? Let these masters of omniscience tell us. Is it not plain to every one that it has risen in vapour, and has been consumed by the heat of the sun? They say, none the less, that even the sun is without heat. What time they lose in words! And see what proof they lean upon to resist what is perfectly plain. Its colour is white, and neither reddish nor yellow. It is not then fiery by nature, and its heat results, they say, from the velocity of its rotation. What do they gain? That the sun does not seem to absorb moisture? I do not, however, reject this statement, although it is false, because it helps my argument. I said that the consumption of heat required this prodigious quantity of water. That the sun owes its heat to its nature, or that heat results from its action, makes no difference, provided that it produces the same effects upon the same matter. If you kindle fire by rubbing two pieces of wood together, or if you light them by holding them to a flame, you will have absolutely the same effect. Besides, we see that the great wisdom of Him who governs all, makes the sun travel from one region to another, for fear that, if it remained always in the same place, its excessive heat would destroy the order of the universe. Now it passes into southern regions about the time of the winter solstice, now it returns to the sign of the equinox; from thence it betakes itself to northern regions during the summer solstice, and keeps up by this imperceptible passage a pleasant temperature throughout all the world.
Let the learned people see if they do not disagree among themselves. The water which the sun consumes is, they say, what prevents the sea from rising and flooding the rivers; the warmth of the sun leaves behind the salts and the bitterness of the waters, and absorbs from them the pure and drinkable particles, thanks to the singular virtue of this planet in attracting all that is light and in allowing to fall, like mud and sediment, all which is thick and earthy. From thence come the bitterness, the salt taste and the power of withering and drying up which are characteristic of the sea. While as is notorious, they hold these views, they shift their ground and say that moisture cannot be lessened by the sun.
Source: Nine Homilies of Hexaemeron (New Advent)