Easton's Bible Dictionary (1897)
Used along with the ashes of a red heifer for the ceremonial cleansing of persons defiled by contact with a dead body (Num. 19).
Smith's Bible Dictionary (1863)
[Purification]
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
WATER OF SEPARA'TION, or WA'TER OF UNCLEAN'NESS, was sprinkled upon a person defiled by contact with the dead. See Num 19.