The highly sensitive Marsh test is used to detect arsenic in food or stomach contents. The sample is mixed with zinc and sulphuric acid, any arsenic present causing the production of arsine gas and hydrogen. The gas is then led through a tube where it is heated strongly, decomposing into hydrogen and arsenic vapour. When the arsenic vapour impinges on a cold surface, a mirror-like deposit of arsenic forms. The same result can be produced by antimony, but the deposit is tested with sodium hypochlorite. Arsenic dissolves in this compound, while antimony does not.
Before the invention of the Marsh test, arsenic had been a popular choice for poisoners because of the difficulty of detecting it. When the test was devised by the English chemist James Marsh in 1832, as a substitute for the less reliable hydrogen sulphide test, deliberate arsenic poisoning became rarer.