James Marsh

From Wikipedia, the free encyclopedia

James Marsh (September 2, 1794- June 21, 1846) was a chemist who invented the Marsh test for detecting arsenic.

While he was most famous for inventing the test for arsenic that bears his name, Marsh was also a skilled and inventive scientist who held the post of Ordnance Chemist at the Royal Arsenal at Woolwich. He developed the screw time fuze for mortar shells and in 1830 the percussion tube. In 1832 the HMS Castor was the first ship to have her guns modified with these innovations. They were not approved for the Army until 1845, when Woolwich began their manufacture -- for coastal artillery only. They became obsolete in 1866. He also worked as an assistant to Michael Faraday at the nearby Royal Military Academy from 1829 to 1846.

In 1832 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. He performed the standard test by mixing a suspected sample with hydrogen sulfide and hydrochloric acid. While Marsh was able to detect arsenic as yellow arsenic trisulfide, when it came to show it to the jury it had deteriorated, allowing the suspect to be acquitted due to reasonable doubt.

Annoyed by this, he developed a much better test. 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. So sensitive was the test that it could detect arsenic for as little as one-fiftieth of a milligram. He first described this test in The Edinburgh Philosophical Journal in 1838.

[edit] References

• Marsh J. (1836). "Account of a method of separating small quantities of arsenic from substances with which it may be mixed". Eddinburgh New Philosophical Journal 21: 229-236..