An aqueous solution of permanganate of potash yields with baryta-water a violet mixture, which afterwards becomes colourless, and deposits a blue precipitate. This retains its colour after washing and drying, but cannot be recommended as a pigment, being liable to suffer in contact with organic substances, which deoxidize and decolourize the manganates and permanganates. 151. _Platinum Blue._ With mercurous nitrate, the platinocyanide of potassium forms a thick smalt blue, and the platinidcyanide a dark blue precipitate. The compound is a mixture of platino- or platinidcyanide of mercury and mercurous nitrate. Upon the presence of the latter the colour seems to depend, for on washing with cold water containing nitric acid, the nitrate is not removed nor the blue affected; but boiling water extracts the nitrate and leaves a white residue. A blue containing mercurous nitrate must necessarily be injured by impure air, and be otherwise objectionable. 152. _Tungsten Blue_ is an oxide formed by the action of various deoxidizing agents on tungstic acid. It remains unaltered in the air at ordinary temperatures, is opaque, and of a blackish indigo-blue colour. As a pigment, there is little to recommend it. 153. _Wood-Tar Blue._ The colours obtained from coal-tar have become household words, and it is not impossible that those from wood-tar may be some day equally familiar. At present wood-tar is comparatively unexplored, but the fact that picamar furnishes a blue is at least as suggestive and hopeful as that transient purple colouration by which aniline was once chiefly distinguished. As aniline is a product of coal-tar, so picamar is a product of wood-tar; and as the former gives a purple with hypochlorites, so the latter yields a blue with baryta-water. Both are distinguished by coloured tests, but there is this advantage in the picamar blue--it is comparatively permanent. Picamar blue is produced when a few drops of baryta-water are added to an alcoholic solution of impure picamar, or even to wood-tar oil deprived of its acid. The liquor instantly assumes a bright blue tint, which in a few minutes passes into an indigo colour. From [Greek: pitta] pitch, and [Greek: kallos] ornament, the blue is named _Pittacal_. The mode of separating pittacal has not been clearly described. Dumas states, that when precipitated in a flocculent state from its solutions, or obtained by evaporation, it closely resembles indigo, and, like it, acquires a coppery hue when rubbed. It is inodorous, tasteless, and not volatile; and is abundantly soluble in acetic acid, forming a red liquid, which, when saturated by an alkali, becomes of a bright blue. It is represented as a more delicate test of acid and alkalis than litmus. With acetate of lead, protochloride of tin, ammonio-sulphate of copper, and acetate of alumina, it yields a fine blue colour with a tint of violet, said not to be affected by air or light, and therefore recommended for dyeing.