Calomel is apparently a colourless, white or brown tasteless compound, Hg2Cl2, used as a purgative and insecticide. It is also called mercurous chloride.
On page 98 of Bryant's life of Macaulay, he says that Macaulay's doctor (Bright) prescribed it. It is referred to in these places
... suddenly addressing the prentices and Guster, to their consternation, "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....
Bleak House by Dickens
The latter was a feeble child, and it was only by prodigious quantities of calomel that Lady Southdown was able to keep him in life at all.
Vanity Fair by Thackeray