Pharaoh's serpent reaction. Here a small pile of mercury thiocyanate is ignited. This causes a rapid exothermic reaction, which produces a large mass of a coiling serpent-like solid. The reaction produces sulfur dioxide and mercury vapour, and the solid mass contains mercuric sulfide and paracyanogen. This compound was formerly sold as an indoor firework, but the toxic nature of the mercury compounds has led to its being banned.