Is it ensure, insure, or assure?
old name for mercurous chloride, 1670s, from French calomel, supposedly (Littré) from Greek kalos "fair" + melas "black;" but as the powder is yellowish-white this seems difficult. "It is perhaps of significance that the salt is blackened by ammonia and alkalis" [Flood].
calomel cal·o·mel (kāl'ə-měl', -məl)
A colorless, white or brown tasteless compound used as a purgative and an insecticide. Also called mercurous chloride.
a very heavy, soft, white, sweetish-tasting halide mineral, formed by the alteration of other mercury minerals, such as cinnabar or amalgams. Calomel is found together with native mercury, cinnabar, calcite, limonite, and clay, at Moschellandsberg, Ger.; Zimapan, Mexico; and Brewster county, Texas, U.S. For detailed physical properties, see halide mineral (table)