Jamieson, Fausset and Brown on Genesis 1:6

Ge 1:6-8. Second Day.

6. firmament--an expanse--a beating out as a plate of metal: a name given to the atmosphere from its appearing to an observer to be the vault of heaven, supporting the weight of the watery clouds. By the creation of an atmosphere, the lighter parts of the waters which overspread the earth's surface were drawn up and suspended in the visible heavens, while the larger and heavier mass remained below. The air was thus "in the midst of the waters," that is, separated them; and this being the apparent use of it, is the only one mentioned, although the atmosphere serves other uses, as a medium of life and light.