Sounds like calomel, or mercurous chloride. It was a very common remedy in 18th c. Heroic Medicine. It was notoriously taken to cure gonorrhea and syphilis , but was commonly used for other infections and as a strong "purgative" laxative as well. It did have some antibiotic properties, but the toxicity to humans was very great. It was also somewhat unstable: just exposure to light could change it to mercury chlorate, a more water-soluble and so much more toxic compound that could kill or make people blind, so calomel was commonly kept in dark glass bottles.

There seems to have been an expectation that the dose should be large enough to have major side effects, if it was to work. Patients were expected to start drooling or salivating when the right amount was reached. In the journal of James Boswell,he writes that after he discovered he had contracted gonorrhea his doctor put him on a very long course of treatment with calomel, during which he was to do very little other than take the drug. Because of the very limited pharma of the time, it seems to have been deployed for all sorts of things. Lewis and Clark handed it out to their men like it was daily vitamins, and in Richard Henry Dana's Two Years Before the Mast, a sailor with an undiagnosed illness is dosed with calomel as a "here , try this" remedy, and dies. Dana and the rest of the crew don't wonder whether the patient died of infection or calomel.