Arsenic is a mineral, it has many legitimate uses, what I’m interested in is White Arsenic.
Back in the day (1800s) Arsenic was very popular for many things including dispatching those you were less fond of gaining it the nickname of ‘inheritors powder’
We forget that in times before modern day toxicology and forensics it was much easier to get away with poisoning. Symptoms of arsenic poison were not dissimilar to those of cholera therefore often arsenic poisoning went undetected.
Arsenic was surprisingly available and people used it for all kinds of weird and wonderful things, rat poison, fly paper, cosmetics - mixed with chalk and vinegar women used it to whiten their faces. It was regularly used to treat syphilis. In fact it was often used as a general cure all, almost any ailment could be potentially treated with a non-lethal dose of arsenic tonic though most doctors had figured out that it shouldn’t be suggested during pregnancy. In addition it was incredibly cheap and available in your local apothecary.
The symptoms of arsenic poisoning were: headaches, vomiting, cramps, blood in urine to mention just a few and the recipient could take from just a few hours up to a few days to die depending on the strength of the dose.
Medical science was expanding during the height of arsenic poisoning but knowledge was not the same across the board. Doctors used milk, sugar and water, more commonly leeches and bleeding to treat it's symptoms.
Before James Marsh and his recognised testing came along in 1836 doctors also had no specific testing they could use to prove arsenic poisoning. They had to rely on their experience and training (which again was of varying standards). Initially it was suggested that white arsenic had a garlic smell to it.
Many writers have favoured poison as a murder weapon, Agatha Christie, in particular, was fond of a good poisoning including arsenic.
There are many famous cases of arsenic poisoning from Pauper to Royalty. I encourage you to get out there and investigate for yourselves.