Strong Poison: Episode Three

💡 Did You Know?

The chemical test that Bunter carries out on the sample of white powder is called the Marsh Test, developed in 1836 by chemist James Marsh. The sample suspected of containing arsenic is mixed in a flat-bottomed flask with hydrochloric or sulfuric acid and some zinc metal, with any gas given off being drawn through a glass tube which is then heated by a flame. If the sample contains arsenic, it is converted to arsenic trihydride gas (AsH3) which is broken down by the heat of the flame to metallic arsenic and hydrogen. A flame is also applied to the end of the glass tube to burn off any hydrogen produced and remove the risk of explosion. If the sample does contain arsenic, then it appears as a metallic stain on the inside of the glass tube. In this case, Bunter also holds a white porcelain bowl to the end of the tube and the metallic stain develops on its rim, proving that the sample does indeed contain arsenic.