Both from the chemical and practical standpoint it is necessary to divide pigments into clearly defined groups, the following classification being adopted on the basis and natural history of the substances concerned:-- (1) Pigments occurring native in a finished condition, and only requiring mechanical preparation to fit them for use as painters’ colours. (2) Pigments which are not ready formed in Nature, but contain some metallic compound as pigmentary material, which requires certain chemical treatment for its full development. (3) Pigments which, in contrast to these two groups, contain only organic, and no inorganic, constituents. This last class comprises all the natural vegetable pigments, together with the large group of colours obtained artificially from tar products, fresh groups of which are being continually introduced. Nowadays, there is no longer any strict line of demarcation between the natural and artificial organic colouring matters, it being possible to produce even those of the vegetable series, such as madder and indigo, by artificial means. Whilst this group of colours exhibits the greatest variety, and is constantly being enriched and increased by the progress of colour chemistry, the case is different with the first group, the natural earth pigments. Here we have chiefly to do with the preparation of materials occurring in Nature, or with bringing about certain chemical results, so that, consequently, the range of variety is far more restricted, and there is little or no possibility of increasing the number of these colours by the manufacture of really new products. The earth colours nevertheless have a high technical and economic importance, on account of their extremely valuable properties, coupled, for the most part, with low cost. If the term “earth colours” were strictly adhered to, the present work would have to be confined to a description of the physical and chemical properties of the various pigments, and of the various means by which they can be brought into suitable condition for use in paints. However, of late, the term has found wider application than formerly, since it has been found practicable to modify (shade) certain of the earth colours by simple operations, and thus considerably increase the range of tones of the substances known as earth colours. The progress of chemical industry has also largely increased the number of the so-called earth colours, certain methods of chemical treatment having enabled substances that are of little use for other purposes, to be employed, in large quantities, as pigments. The application of these--usually cheap--by-products is still further facilitated by the fact that they can be transformed, by a simple chemical treatment, into pigments which are distinguished by their beauty of colour and at the same time possess the great advantages of durability and cheapness.