In 1775 he investigated arsenic acid and its reactions, discovering arseniuretted hydrogen and "Scheele's green" (copper arsenite), a process for preparing which on a large scale he published in 1778.
A piece of bibulous paper, moistened with silver nitrate, is held over the mouth of the tube, and if arsenic be present, a grey or black deposit is seen on the paper, due to the silver nitrate being reduced by the arseniuretted hydrogen.
The arseniuretted hydrogen produced is passed through a tube containing lead acetate paper and soda-lime, and finally through a narrow glass tube, constricted at various points, and heated by a very small flame.
As the arseniuretted hydrogen passes over the heated portion it is decomposed and a black deposit formed.
Arsenic trihydride (arsine or arseniuretted hydrogen), AsH3, is formed by decomposing zinc arsenide with dilute sulphuric acid; by the action of nascent hydrogen on arsenious compounds, and by the electrolysis of solutions of arsenious and arsenic acids; it is also a product of the action of organic matter on many arsenic compounds.