The Collaborative International Dictionary of English v.0.48:

Calomel \Cal"o*mel\ (k[a^]l"[-o]*m[e^]l), n. [Gr. kalo`s
   beautiful + me`las black. So called from its being white,
   though made from a black mixture of mercury and corrosive
   sublimate. Cf. F. calom['e]las.] (Chem.)
   Mild chloride of mercury, Hg2Cl2, a heavy, white or
   yellowish white substance, insoluble and tasteless, much used
   in medicine as a mercurial and purgative; mercurous chloride.
   It occurs native as the mineral horn quicksilver.
   [1913 Webster]