Genesis 1:7

And let it divide between water and water: and it was so. And God made the firmament; and God divided between the water which was under the firmament, and the water above the firmament: and it was so.

As the excessive volume of water bore along over the face of the earth, the earth was by reason thereof "invisible" and "formless." When the Lord of all designed to make the invisible visible, He fixed then a third part of the waters in the midst; and another third part He set by itself on high, raising it together with the firmament by His own power; and the remaining third He left beneath, for the use and benefit of men. Now at this point we have an asterisk. The words are found in the Hebrew, but do not occur in the Septuagint.

The firmament was created on the evening of the second night, just as the heavens came to be on the evening of the first night. But when the firmament came into existence, the covering of clouds that had served for a night and a day in the place of the firmament dissipated. Because [ the firmament ] had been created between the light and the darkness, no darkness remained above it, for the shadow of the clouds was dispelled when the clouds themselves were dispelled. Nor did any of this light remain there, for its alotted measure of time had come to an end and so it sank into the waters that were beneath [ the firmament ].
The wind could not have remained there, either, because it did not even exist there. It was on the first night that Moses said "it hovered" and not on the second night. If the firmament had been created on the first night when [ the wind ] was blowing there could then be some debate. But, since it is not written that [ the wind ] was blowing when the firmament was created, who would say that the wind was there when Scripture does not say so?
After the wind hovered on the first day, manifested its service by its blowing and returned to its stillness, then the firmament came to be. It is evident, therefore, that [ the wind ] neither remained above nor descended below, for how can one seek in any place or spot for something whose very substance only exists at the moment of its service and whose service comes to an end when it ceases to blow? The wind underwent three things on the day of its creation: it was created from nothing, it blew in and through something, and it reverted to being hidden in its stillness.
After the wind had undergone these three things, the firmament was created on the evening of the second day. There was then nothing that rose along with it, because there was nothing that remained above it. It made a separation between the waters that it was commanded to separate, but not between the light, the wind or darkness, for this had not been commanded.
There was no light, therefore, on the first night. On the night of the second and third day, it sank into the waters beneath the firmament and sprang forth as we said [ above ]. But on the fourth day, when the waters were gathered into one place, they say that the firmament was formed and that the sun, the moon, and the stars were formed from the firmament and from fire, and there were places set apart for the lights. The moon would rise in the west of the firmament, the sun in the east, and at the same moment, the stars were dispersed in orderly fashion throughout the entire firmament.

Someone may ask this: Why does the Scripture reduce to a command of the Creator that tendency to flow downward which belongs naturally to water?… If water has this tendency by nature, the command ordering the waters to be gathered together into one place would be superfluous.… To this inquiry we say this, that you recognized very well the movements of the water after the command of the Lord, both that it is unsteady and unstable and that it is borne naturally down slopes and into hollows; but how it had any power previous to that, before the motion was engendered in it from this command, you yourself neither know nor have you heard it from one who knew. Reflect that the voice of God makes nature, and the command given at that time to creation provided the future course of action for the creatures.

But as far as concerns the separation of the waters I am obliged to contest the opinion of certain writers in the church who, under the shadow of high and sublime conceptions, have launched out into metaphor and have seen in the waters only a figure to denote spiritual and incorporeal powers. In the higher regions, accordingly, above the firmament, dwell the better; in the lower regions, earth and matter are the dwelling place of the malignant. So, say they, God is praised by the waters that are above the heavens, that is to say, by the good powers, the purity of whose soul makes them worthy to sing the praises of God. And the waters that are under the heavens represent the wicked spirits, who from their natural height have fallen into the abyss of evil. Turbulent, seditious, agitated by the tumultuous waves of passion, they have received the name of sea, because of the instability and the inconstancy of their movements. Let us reject these theories as dreams and old women’s tales.

Before laying hold of the meaning of Scripture let us try to meet objections from other quarters. We are asked how, if the firmament is a spherical body, as it appears to the eye, its convex circumference can contain the water which flows and circulates in higher regions? What shall we answer? One thing only: because the interior of a body presents a perfect concavity it does not necessarily follow that its exterior surface is spherical and smoothly rounded. Look at the stone vaults of baths, and the structure of buildings of cave form; the dome, which forms the interior, does not prevent the roof from having ordinarily a flat surface. Let these unfortunate men cease, then, from tormenting us and themselves about the impossibility of our retaining water in the higher regions.
Now we must say something about the nature of the firmament, and why it received the order to hold the middle place between the waters. Scripture constantly makes use of the word firmament to express extraordinary strength. The Lord my firmament and refuge. I have strengthened the pillars of it. Praise him in the firmament of his power. The heathen writers thus call a strong body one which is compact and full, to distinguish it from the mathematical body. A mathematical body is a body which exists only in the three dimensions, breadth, depth, and height. A firm body, on the contrary, adds resistance to the dimensions. It is the custom of Scripture to call firmament all that is strong and unyielding. It even uses the word to denote the condensation of the air: He, it says, who strengthens the thunder. Scripture means by the strengthening of the thunder, the strength and resistance of the wind, which, enclosed in the hollows of the clouds, produces the noise of thunder when it breaks through with violence. Here then, according to me, is a firm substance, capable of retaining the fluid and unstable element water; and as, according to the common acceptation, it appears that the firmament owes its origin to water, we must not believe that it resembles frozen water or any other matter produced by the filtration of water; as, for example, rock crystal, which is said to owe its metamorphosis to excessive congelation, or the transparent stone which forms in mines. This pellucid stone, if one finds it in its natural perfection, without cracks inside, or the least spot of corruption, almost rivals the air in clearness. We cannot compare the firmament to one of these substances. To hold such an opinion about celestial bodies would be childish and foolish; and although everything may be in everything, fire in earth, air in water, and of the other elements the one in the other; although none of those which come under our senses are pure and without mixture, either with the element which serves as a medium for it, or with that which is contrary to it; I, nevertheless, dare not affirm that the firmament was formed of one of these simple substances, or of a mixture of them, for I am taught by Scripture not to allow my imagination to wander too far afield. But do not let us forget to remark that, after these divine words let there be a firmament, it is not said and the firmament was made but, and God made the firmament, and divided the waters. Genesis 1:7 Hear, O you deaf! See, O you blind!— who, then, is deaf? He who does not hear this startling voice of the Holy Spirit. Who is blind? He who does not see such clear proofs of the Only begotten. Let there be a firmament. It is the voice of the primary and principal Cause. And God made the firmament. Here is a witness to the active and creative power of God.

The matter was separated by the interposition of the firmament so that the lower matter is that of bodies and the higher matter that of souls.

The waters were divided so that some were above the firmament and others below the firmament. Since we said that matter was called water, I believe that the firmament of heaven separated the corporeal matter of visible things from the incorporeal matter of invisible things.

Firmament: Not that in which the stars are set, but the part of the atmosphere where the clouds are collected, and which has received the name firmament from the firmness and density of the air. "For a body is called firm," that is dense and solid, "thereby differing from a mathematical body" as is remarked by Basil (Hom. iii in Hexaem.).

Above the firmament: The waters above the firmament must rather be the vapors resolved from the waters which are raised above a part of the atmosphere, and from which the rain falls. As to the nature of these waters, all are not agreed. Origen says (Hom. i in Gen.) that the waters that are above the firmament are "spiritual substances." Wherefore it is written (Psalm 148:4): "Let the waters that are above the heavens praise the name of the Lord," and (Daniel 3:60): "Ye waters that are above the heavens, bless the Lord." To this Basil answers (Hom. iii in Hexaem.) that these words do not mean that these waters are rational creatures, but that "the thoughtful contemplation of them by those who understand fulfils the glory of the Creator." Hence in the same context, fire, hail, and other like creatures, are invoked in the same way, though no one would attribute reason to these. We must hold, then, these waters to be material, but their exact nature will be differently defined according as opinions on the firmament differ.