First and foremost, allow me to apologize for entering a place where only the adepts of chemistry should tread – however, I’ve felt much relief when I found out that this forum has already allowed questions similar to mine; i.e. one coming from somebody who writes a novel with a plot point involving chemistry… who tries not to be, but still comparatively is a chemical ignoramus… yet who hopes to make that plot point plausible enough that, should a chemist read it, s/he will not groan and toss the book across the room.
The question revolves – as they tend to in fiction – around a dangerous chemical substance that would be lethal to people, either by being highly poisonous, or e.g. by its volatile nature. The main problem is that the substance in question would have to either be known in early 18th century, or to occur naturally. (As a last resort, it might be discovered or otherwise obtained by a unique character, someone of Leonardo da Vinci’s nature, presumably forgotten later on – an obscure genius, too early for his time, etc… although I would prefer to avoid such a crutch.)
Even better, it would work relatively fast when “triggered” (I’m assuming either by reaction, or by being absorbed, e.g. by touch) – in other words, when the right time comes, a character could somehow use the substance’s destructive nature against another character, or trick said opponent into “triggering” that effect.
Ideally, it would also be something that could be either easily concealed, or made to appear as if it was something else, to trick a casual observer (e.g. by being mixed with another metal while retaining its dangerous characteristics, or by being shaped into seemingly benign forms, such as coins, etc.)
Here is the direction of my thoughts, to give some examples.
- If isolated potassium was accessible in early 18th century, it would be perfect for the scene, as it would just take some water to trigger the desired effect, i.e. to cause flames or even an explosion... Unfortunately, as far as I understand, it would be far too early for anyone to have potassium (or any other alkali metal that reacts with water…?) in that time period… or could the idea be somehow made viable?
- Radioactive elements: but, again, those would be too early, would they not? Or would it be possible for an early 18th century character to somehow amass enough of a naturally-occurring radioactive element (radioactive enough to actually act quickly against the enemies – e.g. once it was released from a lead container)… and if so, which one could it be? (I’m asking the question in this forum even though there is a separate one on nuclear chemistry, since I don’t want to spam many places…)
- Poisonous (and fast-acting) substances: another possibility, and, I’m assuming, safer than the previous ones – yet, once again, would a character in that time period be able to obtain such a substance (and/or amass enough of it)?
Or could there be some entirely different direction, one of which I have not thought? (And I have no doubt there are many such ones…