The highly-toxic substance was in fact more of a poison than a cure, though its presence allowed for the preservation of Bischoff's corpse.
And yet, DNA testing has revealed that a mummified body found beneath a Swiss church is the Foreign Secretary's seven-times-great-grandmother. We didn't know where we were going and whether we would gain anything.
Anthropologist Gerhard Hotz, curator at Basel's Natural History Museum, said identifying her took two years of thorough research, having started with nearly no information on who the mummy was. The woman is the great-great-great-great-great-great-great grandmother of Johnson, researchers said.
The mummy had been uncovered once before - in 1843, during building work at the church.
The DNA was a 99.8% match, leading historians to identify the body as that of Bischoff.
She married a church minister, and lived for much of her adult life in Strasbourg.
Genealogists were able to trace Anna Catharina's descendants through birth, marriage, and death records.
Once Anna Catharina's husband passed away, she returned to her hometown and started an intense treatment to cure the disease, but to no avail.
One of her surviving children, Anna Katharina Gernler (1739 - 1776), married German aristocrat Christian Hubert Baron Pfeffel von Kriegelstein.
Five generations of von Pfeffels later, Marie Luise von Pfeffel marryied Stanley Fred Williams.
Boris Johnson has yet to comment on the discovery of his long lost relative, but he is certainly aware of the von Pfeffel connections, having once told the BBC programme Who Do You Think You Are? that they were "posh toffs".
She died in 1787 at the age of 68, from mercury poisoning, scientists believe.
Johnson tweeted that he was "very excited to hear about my late great grand "mummy" - a pioneer in sexual health care. And then entered Boris Johnson - what more do you want?"