The heteropolytungstic acids, especially certain of the phosphotungstic and silicotungstic acids, are photooxidizing agents with properties that suggest their use in photographic processes. The chemical and physical properties of the substances are described, and simple demonstrations of three new photographic processes based upon phosphotungstic acid are outlined. These comprise a printing out process producing images in pigments of basic dyes; phototropic, and therefore reusable, printing out materials; and the development of a latent image.
LYMAN CHALKLEY, "Experimental Photographic Systems Based on Phosphotungstic Acid," J. Opt. Soc. Am. 44, 699-702 (1954)