=Gold Compounds.=—Gold is now very little used in preparing colours. The compound used for this purpose is gold chloride, AuCl₃, which is obtained by heating gold with hydrochloric acid, and adding nitric acid in small quantities until all the metal is dissolved. By careful evaporation of the yellow solution gold chloride is obtained in brownish yellow crystals, which easily dissolve in water. The compounds of molybdenum, vanadium and uranium are less used than those of gold, yet these metals find a special use in the preparation of colours for porcelain painting and for colouring glass, for which they are of great importance. The preparation of their compounds from the raw materials is complicated and not remunerative to the colour maker; they should be obtained from chemical works. * * * * * In the foregoing, the most important compounds of inorganic origin used in making colours have been briefly described, less in order to teach the methods for their preparation than to give the manufacturer the means of learning their properties. Since the great development of chemical industries during the last decades it is more advantageous for the colour maker in most cases to draw his supply of these substances from works of good reputation than to make them himself; only in the case of a few substances, which are sold at unreasonably high prices, will it be profitable for him to prepare them himself. CHAPTER VI. THE MANUFACTURE OF MINERAL PIGMENTS. By mineral pigments we understand those which consist of compounds of metals with elements such as sulphur, chlorine and iodine, or with compound radicals, such as cyanogen, or of metallic salts. Looking at the classification of mineral pigments from the chemical standpoint, a grouping according to the constituents would appear preferable, and we should have groups of colours consisting of metallic oxides, sulphides, salts, etc. In such a division of pigments, according to their constituents, no regard would be taken of the colour of the pigment. The white zinc oxide would be placed in the same group with yellow lead oxide and red lead oxide; and red mercury sulphide and yellow cadmium sulphide would fall into the same group of the sulphur compounds. The chemical composition of a pigment is of less importance to the colour manufacturer than its shade, and it therefore appears to us more reasonable to prefer to classify mineral pigments according to similarity of colour. We shall therefore place all those mineral pigments together which possess the same colour.