=Tessié du Motay’s Blue.=—The following is the method of preparation: 10 parts of sodium tungstate, 8 parts of tin crystals, 5 parts of yellow prussiate, and 1 part of ferric chloride are dissolved separately and the solutions mixed. The precipitate is washed and exposed to light in thin layers. The blue colour develops in the course of several days. According to the discoverer, this pigment consists of a compound of tungsten oxide with a double cyanide of iron and tin; in physical properties it is similar to good Prussian blue, but is distinguished by greater durability on exposure to light and also by a much higher cost. Up to the present this pigment has not been made commercially. =Molybdenum Blue.=—This fine but costly pigment is obtained by mixing a solution of sodium molybdate with a solution of stannous chloride. A blue precipitate is formed, which, after washing and drying, may be used as an artists’ colour. It is distinguished by great durability. A good shade of molybdenum blue can be obtained with greater certainty by adding finely-powdered tin and a few drops of hydrochloric acid to a solution of pure molybdic acid. In composition it is a mixture of stannic molybdate and the blue modification of molybdenum oxide. CHAPTER XXVIII. GREEN MINERAL PIGMENTS—GREEN COPPER PIGMENTS. =Copper Carbonate.=—In nature two salts occur containing copper, carbonic acid, and water in different proportions—one, the mineral malachite, has a fine emerald green colour; the other, azurite, is sky blue. Both minerals are ground and levigated, and brought into commerce as mountain green and mountain blue. Compounds of copper with carbonic acid, made artificially, are used under different names as pigments. The terms mountain blue and mineral blue are applied indiscriminately to various copper compounds; as mountain blue, or Brunswick green, pigments come into commerce containing varying quantities of copper carbonate (made by precipitating a copper salt with soda) mixed with barytes, zinc white, or another white pigment, in order to obtain paler and deeper shades. The poisonous Scheele’s green is also given the same name as that under which copper carbonate is sold, _viz._, mineral green, so that the composition of this colour can only be decided by analysis. A green pigment, similar in composition to malachite, is obtained by precipitating copper sulphate with soda in the cold and washing quickly. If the precipitate is left in contact with the liquid, its colour changes and it is converted into a blue mass similar in properties to ground malachite. A green precipitate is obtained with more certainty by precipitating a boiling solution of copper sulphate with soda solution. Both green and blue copper carbonate are pigments of little depth; as a rule ground and levigated malachite and azurite have a greater intensity of colour than the artificial pigments.