Job 40:18

“His bones [are as] strong pieces of brass: his bones [are] as bars of iron.] Than which nothing is stronger. The repetition is made for greater illustration and confirmation; but what is said is not applicable to the elephant, whose bones are porous and rimous, light and spongy for the most part, as appears from the osteology f1475 of it; excepting its teeth, which are the ivory; though the teeth of the river horse are said to exceed them in hardness f1476; and artificers say f1477 they are wrought with greater difficulty than ivory. The ancients, according to Pausanias f1478, used them instead of it; who relates, that the face of the image of the goddess Cybele was made of them: and Kircher…”

“strong--rather, "tubes" of copper [UMBREIT].”

“His bones are as strong pieces of brass - The circumstance here adverted to was remarkable, because the common residence of the animal was the water, and the bones of aquatic animals are generally hollow, and much less firm than those of land animals. It should be observed here, that the word rendered “brass” in the Scriptures most probably denotes “copper.” Brass is a compound metal, composed of copper and zinc; and there is no reason to suppose that the art of compounding it was known at as early a period of the world as the time of Job. The word here translated “strong pieces” (אפיק 'âphı̂yq) is rendered by Schultens “alvei - channels,” or “beds,” as of a rivulet or stream; and by…”

“Job 7:12; Isa 48:4 Job 40:19”