Hastings' Dictionary of the Bible (1898)
SERPENT, BRAZEN. Num 21:9. As a punishment for the murmuring of the Israelites, God sent into the midst of the camp a venomous serpent, called "fiery," probably, from the burning which followed its deadly bite. There are many species of such dangerous serpents still found in the wilderness of Sinai, the various kinds of which, or perhaps some particular species, may here be intended. The destruction of life was fearful, and the people entreated Moses to intercede for their deliverance.
To test the sincerity of their penitence, Moses was commanded to make a serpent of brass resembling the serpents which were among them, and put it upon a pole, that it might be seen from all parts of the camp, and then whoever was bitten should be healed by simply looking at the brazen figure; and it was accordingly done, and all the promised effects followed. This passage of history is alluded to by our Saviour as an illustration of the work he came to do. John 3:14-15.