"Experiments on Arsenic... experiments... are as early... as 1764... [T]he paper... contains an elaborate enquiry into the differences between regulus of Arsenic (Metallic ), white Arsenic (Arsenious Acid, AsO3), and Arsenical Acid (Arsenic Acid, AsO5). The properties... are described with no little accuracy. ...Cavendish ...held arsenic acid to be "more thoroughly deprived of its phlogiston" than arsenious acid; and the latter to bear a similar relation to metallic arsenic. ...[Equivalently] arsenic acid contains more oxygen than arsenious acid, and the latter more than metallic arsenic, which we know to be the case. The paper, is otherwise remarkable for its speculations on the nature of the "red fumes," (, produced by the action of the air on ) which attended the action of nitric acid on arsenious acid, and for its discussion of the theory of the solution of metals in acids, and the reduction of the former by heat and inflammable matter."