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.

...

BUNTHORNE: It is a wild, weird, fleshy thing; yet very tender, very yearning, very precious. It is called, "Oh, Hollow! Hollow! Hollow!"

PATIENCE: Is it a hunting song?

BUN. A hunting song? No, it is not a hunting song. It is the wail of the poet's heart on discovering that everything is commonplace. To understand it, cling passionately to one another and think of faint lilies.

[They do so as he recites:]

             "OH, HOLLOW! HOLLOW! HOLLOW!"

          What time the poet hath hymned
          The writhing maid, lithe-limbed,
               Quivering on amaranthine asphodel,
          How can he paint her woes,
          Knowing, as well he knows,
               That all can be set right with calomel?

          When from the poet's plinth
          The amorous colocynth
               Yearns for the aloe, faint with rapturous thrills,
          How can he hymn their throes
          Knowing, as well he knows,
               That they are only uncompounded pills?

          Is it, and can it be,
          Nature hath this decree,
               Nothing poetic in the world shall dwell?
          Or that in all her works
          Something poetic lurks,
               Even in colocynth and calomel?
                        I cannot tell. 

--G&S, Patience

Lots of other lovely Gilbertian words there, too :-)