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,” etc. (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 Mat 5:22. James has much in common with the Sermon on the Mount (Pro 16:27).