Born in Kent, he was working as a labourer in Woolwich in the late 1810s and early 1820s, before joining the Royal Artillery.

[4] In 1833 Marsh was called as a chemist by the prosecution in a murder trial, wherein a certain John Bodle was accused of poisoning his grandfather with arsenic-laced coffee.

Marsh performed the standard test by mixing a suspected sample with hydrogen sulfide and hydrochloric acid.

He combined a sample containing arsenic with sulfuric acid and arsenic-free zinc, resulting in arsine gas.

The gas was ignited, and it decomposed to pure metallic arsenic which, when passed to a cold surface, would appear as a silvery-black deposit.