Jamieson, Fausset and Brown on James 3:6

6. Translate, "The tongue, that world of iniquity, is a fire." As man's little world is an image of the greater world, the universe, so the tongue is an image of the former [Bengel].

so--omitted in the oldest authorities.

is--literally, "is constituted." "The tongue is (constituted), among the members, the one which defileth," &c. (namely, as fire defiles with its smoke).

course of nature--"the orb (cycle) of creation."

setteth on fire ... is set on fire--habitually and continually. While a man inflames others, he passes out of his own power, being consumed in the flame himself.

of hell--that is, of the devil. Greek, "Gehenna"; found here only and in Mt 5:22. James has much in common with the Sermon on the Mount (Pr 16:27).