Albert Barnes Commentary Job 37:18

"Canst thou with him spread out the sky, Which is strong as a molten mirror?" — Job 37:18 (ASV)

Hast thou with him spread out the sky? - That is, were you employed with God in performing that vast work, so that you can explain how it was done? Elihu here speaks of the sky as it appears, and as it is often spoken of, as an expanse or solid body spread out over our heads, and as sustained by some cause which is unknown. Sometimes in the Scriptures it is spoken of as a curtain (Isaiah 40:22); sometimes as a “firmament,” or a solid body spread out (Genesis 1:6–7, Septuagint); sometimes as a fixture in which the stars are placed (Isaiah 34:4); and sometimes as a scroll that may be rolled up, or as a garment (Psalms 102:26).

There is no reason to suppose that the true cause of the appearance of an expanse was understood at that time. Probably, the prevailing impression was that the sky was solid and was a fixture in which the stars were held. Many of the ancients supposed that there were concentric spheres, transparent yet solid. They believed these spheres revolved around the earth, carrying the heavenly bodies with them. In one sphere, they supposed, was the sun; in another, the moon; in another, the fixed stars; and in another, the planets. It was the harmonious movement of these concentric and transparent orbs that, according to their supposition, produced the “music of the spheres.”

Which is strong - Firm, compact. Elihu evidently supposed that it was solid. It was so firm that it was self-sustained.

And as a molten looking-glass - As a mirror that is made by being fused or cast. The word “glass” is not in the original; the Hebrew denotes simply “seeing,” or a “mirror” (ראי r^e'ı̂y). Mirrors were commonly made of plates of metal highly polished; see the notes at Isaiah 3:23; compare Wilkinson’s Manners and Customs of the Ancient Egyptians, vol. iii. p. 365. Ancient mirrors were so highly polished that in some which have been discovered at Thebes the luster has been partially restored, though they have been buried for many centuries. There can be no doubt that the early apprehension in regard to the sky was that it was a solid expanse, and that it is often so spoken of in the Bible.

There is, however, no direct declaration that it is so. Whenever it is so spoken of, it is to be understood as popular language, as we still speak of the rising or setting of the sun, though we know that the language is not philosophically correct. The design of the Bible is not to teach science but religion, and the speakers in the Bible were allowed to use the language of common life - just as scientific men in fact do now.