Hi @grasswind, thanks for your question. You've hit the bullseye of why homeopathy causes so many to get hot under the collar.
Homeopathic preparation involves grinding a very tiny amount of the original substance (mercury in this case) in a process called trituration. The triturated mixture is then REPEATEDLY DILUTED in water and shaken up (succussed) until there is NO PARTICLE OF THE ORIGINAL SUBSTANCE LEFT ie. beyond Avogadro's number (https://www.thoughtco.com/definition-of-avogadros-number-604379). This makes the remedy toxin free obviously as no actual mercury is left in the mixture.
You might suppose that this process leaves nothing but water with all the mercury disappeared (and many argue it does) but it appears not so. Hahnemann himself supposed it was the energetic imprint of the original substance transferred to the water, but scientists are still arguing about it.
If you'd like to know more of the science arguments against and for then a good place to start is at https://www.hri-research.org/.
RE: Boris Johnson's grandma, mercury, and the case for Homeopathy