Bereishit 1:7 "G-d said, 'Let there be a firmament in the midst of the waters, and let it divide between water and water.'
7 G-d made the firmament, and He divided between the waters that are below the firmament, and between the waters that are above the firmament, and it was so."
At the beginning of the second day, the firmament (rakia) existed, but it was immersed in water just like the earth. Hashem then commanded, "Let the firmament become stable!" As a result, the waters above the firmament were divided from the waters below the firmament. The water above the firmament evaporated from the heat of the firmament which is fire, and this water later condensed to form rain. (Bereishit Rabbah 4:2)
Upon completing the firmament, Hashem named it shamayim (heaven). The name shamayim reveals the secret of the substances from which it was fashioned - aish (fire) and mayim (water), combining to form the composite, shamayim (heaven). (Bereishit Rabbah) Fire represents the Attribute of Justice. Water stands for the quality of Mercy. The heaven which is the residence of Hashem's glory is comprised of a combination of these two attributes.
At this time, all of creation consisted of water. G-d then commanded that a firmament be made, dividing this water. Part was on top, another part on the bottom, and the firmament was in the middle.
The distance between the earth and the firmament is the same as between the firmament and the upper waters. (Tanchuma)
This teaches that the upper waters are suspended in space through the power of G-d's word. (Ralbag; Abarbanel) Unlike the waters that rest on the ground, they float high above the firmament.
The "water" above the firmament is not a physical liquid, but an ethereal fluid which we call "spiritual substance." (Alshich) Still, we must believe in a sense this is actually water. King David thus sang, "Praise Him O you heaven of heavens, and you waters that are above the heavens" (Tehillim 148:4; Zohar Chadash 10d; Toledot Yitzchak
The waters of purgatory become salty from the heat of the heavens. King David noticed that when water is left on the fire, it becomes more and more salty.
One stream emerges from the abyss, another comes from purgatory, and both appear to be about to meet. At the last moment, the firmament comes between them, and the waters of purgatory return to their place, and do not poison the waters of the depths. But it is impossible that none of the water from purgatory escape, and it is for this reason that there are some springs that are bitter and noxious. (Zohar Chadash 11a; 15b) The springs of Eretz Yisrael, however, all yeild healthy water, and some also have curative powers.
Although heaven was created on the first day, the firmament was created on the second. This firmament is what Yechezkel saw over the head of the angels called Chayot. (Pirkei Rabbi Eliezer; Yalkut) This firmament has a brilliant light prepared for the righteous to enjoy in the Olam Haba (World to Come), as it is written, "The enlightened shall shine like the glow of the firmament" (Daniel 12:3).
Source: Torah Shleimah; Midrash Says; Meam Loez
