Charles Spurgeon Commentary Numbers 19:10

"And he that gathereth the ashes of the heifer shall wash his clothes, and be unclean until the even: and it shall be unto the children of Israel, and unto the stranger that sojourneth among them, for a statute for ever." — Numbers 19:10 (ASV)

That was the remedy ordained by the Lord for purifying the defiled; now notice what made this remedy so necessary.