And Eleazar—said—This is the ordinance of the law— i.e. "This is a law to be observed hereafter by all who go to war." The law before was, that he, who touched a dead body should be purified with water; but nothing had been hitherto said of him who killed a man in war; nor of the spoil, concerning which Moses here prescribes two sorts of purifications, the fire for metals, and the water for all combustible matters. Purifying by water was a rite of the earliest antiquity, (see Genesis 35:2.) and the use of fire in purification appears to have been not much less ancient. Hence Homer makes Ulysses call for sulphur and fire, to fume the house wherein the riotous suitors had been killed, Odyss. b. xxii. v. 518 of Pope's translation; who, in the note upon the place, observes, that Job 18:15 seems to refer to this same custom; brimstone shall be scattered upon his habitation. Livy mentions this practice among the Romans; Habet et in religionibus locum ad expiandas suffitu domus, lib. xxx. cap. 15. The natural reason why fire is proper for that use is assigned by Ovid, Fast. lib. iv. ver. 785.
Omnia purgat edax ignis, vitiumque metallis Excoquit.
We may observe, that it is only said, go through the fire, not that it should be melted. See 1 Corinthians 3:13. For more upon this subject, the learned reader is referred to the works of Scacchi, Bonfrere, and Spencer.