The foregoing are only a few examples of brown or red-brown earth colours. In all these minerals the pigmentary principle is iron, in combination either with oxygen alone (ferric oxide), with oxygen and water (ferric hydroxide), or silica compounds (ferric silicate), and always associated with certain quantities of other metallic oxides, especially alumina and manganese oxide. Although but few of these minerals have gained any special reputation as pigments, there is no doubt that similar minerals, which are certain to occur in or near many deposits of iron ores, could equally well be used for that purpose. There is no need to emphasise that the discovery of such a mineral would be a very valuable find, and that the products obtainable therefrom could be utilised to great advantage. The testing of a mineral for its suitability as pigment is a very simple matter, all that is required being to subject a small quantity to the same treatment that is applied to the earth colours on a large scale. For this purpose a few pounds of the mineral are levigated, and the residue is dried. To ascertain the tones obtainable by calcination, small samples--of about 100 grms.--are placed in crucibles, and gradually heated in a furnace. When the masses have attained a sufficient temperature, the samples are taken out of the furnace, at intervals of ten minutes, and left to cool. It will then not be difficult to decide whether the mineral is at all suitable for the purposes of the colour-maker; and if so, these tests afford at once an indication of the temperature and time the mineral must be heated in order to obtain pigments of definite tones. COLOGNE EARTH (COLOGNE UMBER) The application of the term “umber” to this earth can only have been based on a certain similarity in colour to true umber. In chemical composition, however, the two are quite different, Cologne earth really consisting of a mixture of humic substances. It is well known that the rotted wood found in the interior of decaying trees is often a handsome brown colour; and all woody matter, after lying a very long time, finally acquires this colour, owing to the transformation of the wood into dark-coloured compounds richer in carbon. This effect can be seen on the large scale, in Nature, in the case of coal, brown coal and peat. Now Cologne earth consists of a brown-coal mould, dark brown in colour, of earthy character and of such low cohesive power that it crumbles with ease. Owing to this character, Cologne earth can be easily ignited by the flame of a candle, and then burns with a strong, smoky flame, leaving very little ash and disseminating the peculiar bituminous smell given off when brown coal is burned.