Albert Barnes Commentary Job 40:18

"His bones are [as] tubes of brass; His limbs are like bars of iron." — Job 40:18 (ASV)

His bones are as strong pieces of brass - The circumstance referred to here was remarkable, because the common dwelling place 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 noted here that the word translated “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 as early as the time of Job.

The word translated here as “strong pieces” (אפיק 'âphı̂yq) is translated by Schultens as “alvei—channels,” or “beds,” as of a rivulet or stream; and by Rosenmuller, Gesenius, Noyes, and Umbreit, as “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 it is appropriate to retain that meaning here. The meaning, then, would be that his bones were so firm they seemed to be made of solid metal.