Because arsenic is a toxic metal (as opposed to some kind of biological compound) t precipitates out…

Because arsenic is a toxic metal (as opposed to some kind of biological compound) t precipitates out of liquids pretty intact with the right tests, and after the invention of the Marsh test in the 1830s there was actually a flurry panic about Arsenic Murders. Prior to that they WERE undetectable (arsenic poisoning presents very much like cholera), and arsenic was known as “inheritance powder” half…