A commonly used laxative in the bad old days. Contains mercury, a serious potential hazard.
"“if I am told by the doctor that calomel or castor-oil is good for me, I may naturally ask what is calomel, and what is castor-oil. I may wish to be informed of that, before I dose myself with either or with both. "

Calomel \Cal"o*mel\, n. [Gr. ????? beautiful + ????? 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.