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.
Copper arsenite forms the basis of a number of once valuable, but very poisonous, pigments.
Scheele's green is a basic copper arsenite; Schweinfurt green, an aceto-arsenite; and Casselmann's green a compound of cupric sulphate with potassium or sodium acetate.
Arsenides, Arsenites, &c. - Several iron arsenides occur as minerals; lolingite, FeAs 2, forms silvery rhombic prisms; mispickel or arsenical pyrites, Fe2AsS2, is an important commercial source of arsenic. A basic ferric arsenite, 4Fe2O3 As2O3.5H 2 O, is obtained as a flocculent brown precipitate by adding an arsenite to ferric acetate, or by shaking freshly prepared ferric hydrate with a solution of arsenious oxide.
The prismatic variety of the oxide can be obtained by crystallization from a saturated boiling solution in potassium hydroxide, or by the crystallization of a solution of silver arsenite in nitric acid.