Job 40:18 - Albert Barnes' Notes on the Bible

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 Rosenmuller, Gesenius, Noyes, and Umbreit, “tubes” - supposed to allude to the fact that they seemed to be hollow tubes of brass. But the more common meaning of the word is “strong, mighty,” and there is no impropriety in retaining that sense here; and then the meaning would be, that his bones were so firm that they seemed to be made of solid metal.