Fiery serpents, גְחָשִׁים שְׂרָפִים. Nachash is the ordinary word for serpent. The word saraph which seems to mean "burning one," stands (by itself) for a serpent in Numbers 21:8, and also in Isaiah 14:29; Isaiah 30:6.
In Isaiah 6:2, Isaiah 6:6 it stands for one of the symbolic beings (seraphim) of the prophet's vision. The only idea common to the two meanings (otherwise so distinct) must be that of brilliance and metallic luster.
It is commonly assumed that the "fiery" serpents were so called because of the burning pain and inflammation caused by the bite, after the analogy of the πρηστῆρες and καύσωνες of Dioscorus and AElian.
But is hardly possible that Isaiah should have used the same word in such wholly dissimilar senses, and it is clear from comparison with Ezekiel's vision of the cherubim (Ezekiel 1:7) that the saraph of Isaiah 6:2 was so called from the burnished luster of his appearance.
Even our Lord himself is described in the Apocalypse as having in the highest degree this appearance of glowing brass (Revelation 1:15; Revelation 2:18). It is further clear that the saraph was so named from his colour, not his venom, because when Moses was ordered to make a saraph he made a serpent of brass (or rather copper), with the evident intent of imitating as closely as possible the appearance of the venomous reptile.
We may conclude then with some confidence that these serpents were of a fiery red colour, resembling in this respect certain very deadly snakes in Australia, which are known as "copper snakes." Travelers speak of some such pests as still abounding in the region of the Arabah, but it is quite uncertain whether the fiery serpents of that special visitation can be identified with any existing species.