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The water of separation - The water in which the ashes of the red heifer were mingled; See Numbers 8:7; (note), Numbers 19:2; (note), etc. Garments, whether of cloth or skins, were to be washed. Gold, silver, brass, iron, tin, and lead, to pass through the fire, probably to be melted down.

The Israelites had to purify themselves according to the law, and to abide without the camp seven days, though they had not contracted any moral guilt, the war being just and lawful, and commanded by God. Thus God would preserve in their minds a dread and detestation of shedding blood. The spoil had been used by Midianites, and being now come into the possession of Israelites, it was fit that it should be purified.