It may be worth pointing out here that ultramarine should not be used with any other colours which have a tendency to be acid, as sooner or later the colour will be destroyed. It should also not be used with lead or copper pigments, as the sulphur it contains tends to react on those metals; forming the black sulphides, thus leading to the ultimate discoloration of the mixture.--(_Chemical Trade Journal_). CHAPTER IV. BROWNS. Brown colouring matters are obtained from all three kingdoms--the animal, the vegetable, and the mineral--but in greatest abundance from the last named. The natural mineral brown pigments afford almost every variety of tint, and being largely composed of silica and metallic oxides they are remarkably permanent. ASPHALT OR BITUMEN.--These names are applied to a variety of black or brown resinous matters found in many parts of the world in a mineralised state, though derived originally from organic sources. The “Bitumen of Judæa” is supposed to be found on and around the Dead Sea, but the bulk of the product going by that name really comes from Trinidad. All kinds of asphalt have a pungent and peculiar smell, melt at a low temperature, are very combustible, and while dissolving in turpentine, and more readily in coal tar naphtha, are insoluble in water and in alcohol. Very little asphalt is now used as a pigment, but it continues to find a limited application in varnish making, notwithstanding the tendency of varnishes containing it to suffer from minute cracks with the lapse of time. BISTRE.--This pigment is used exclusively in water-colour painting, for which purpose it affords a fine warm yellowish tinted brown. It is of vegetable origin, being prepared from the soot which is deposited in the flues leading from fireplaces which consume wood fuel. Every wood, however, does not afford an equally good sample of bistre, and beech occupies the foremost rank in this respect. The brightest and blackest soot is selected, and after careful grinding and sifting through a very fine sieve, it is repeatedly stirred up, for several hours at a time, in a series of clean hot waters, the object of which is to dissolve out all traces of tarry and other soluble matters, which very seriously affect its permanence, being oxidised on exposure to air and light, and thus weakening the tint. The washing is therefore a matter of the very first importance. The solid pigment is allowed to settle out of each wash water, and is collected and dried, being mixed with a small proportion of gum water to give cohesion. The drying is effected in a stove room. BONE BROWN.--This unimportant pigment is simply underdone bone black (see p. 6), and is obtained by stopping the calcination of the bones at a point which falls short of thorough charring. In consequence it contains a proportion of unaltered animal matters, which sooner or later may undergo decomposition, and prejudicially affect the painting.