How did the Marsh test detect trace amounts of arsenic?

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A labeled diagram of the periodic table entry for arsenic, showing its atomic number 33, symbol As, atomic weight 74.921595, electron configuration, and classification as an other nonmetal. Additional icons indicate arsenic is solid at room temperature, has a rhombohedral crystal structure, and weakly acidic oxides. — arsenic Properties of arsenic.

The Marsh test detects trace amounts of arsenic by first converting the element into arsine gas. This gas is then subjected to thermal decomposition, which causes the arsenic to deposit as a characteristic black "mirror" on the interior of a narrow glass tube.1