/kal"euh mel', -meuhl/ , n. Pharm.
a white, tasteless powder, Hg 2 Cl 2 , used chiefly as a purgative and fungicide. Also called mercurous chloride .
[ 1670-80; calomelas coined from Gk kaló ( s ) fair + mélas black; allegedly so called because its original preparation involved turning black powder into white ]