We know that Newton's hair samples showed high level of mercury and of course he used mercury like crazy in his alchemy experiments (as did many/all alchemists not just in the west but also China) and in fact Newton had some concern about this or at least thought it affected his hair color (he made some comment about this specifically). The dangers of mercury were not unknown then and by the 19th century they must have been fairly well established although mercury continued to be prescribed as a medicine (and dental filling material as an amalgam) during the US civil war and even into the 20th century for syphilis although by then it was used more topically. Even now I think Mercurochrome is still used as a topical disinfectant in some countries.
We know also that Faraday used mercury so much that he believed, quite reasonably, that it was affecting his cognition/memory. Now, mercury used medicinally is quite different than using it in electrical experiments as Faraday used it -- I believe the first electrical motor of Faraday involved a pool of the element.
Mercury is a good conductor and it also is probably at that time the only conductor that was liquid at room temperature. Was mercury's wide use in experiments due to its physical properties or did it retain some of the glamour from the days of alchemy -- did scientists like Faraday believe it had some special properties beyond the measurable physical properties just as Newton did?