Crude ochres should first be assayed for actual colouring matter and grit or refuse. This can be done by a kind of levigation method: 200 grains of the crude ochre are crushed in a mortar; the grinding must not be too well done, or otherwise faulty results will be obtained. The crushed ochre is put into a tall conical glass; a long glass funnel passes to the bottom of the glass, and the whole is arranged in a large glass basin or dish. A current of water is now caused to flow down the glass funnel; this washes the fine particles of ochre away from the grit, and they are carried over the sides of the glass into the dish. Here they are allowed to settle, and are collected and weighed after drying, an operation which gives the amount of ochre in the crude material. Levigated and prepared ochres can be tested for colour and covering power, by the usual methods. These are the most important points about ochres to which attention should be paid. ORPIMENT.--Orpiment, king’s yellow, or trisulphide of arsenic, is a lemon or orange-yellow coloured substance, found native in Hungary, the Hartz, and other places. The finest samples used by artists (golden orpiment) come from Persia. The commercial article is artificially prepared for use as a pigment in the following way:-- A mixture of arsenious acid and sulphur is placed in an iron subliming-pot, similar to those used in the preparation of crude white arsenic. The mixture is then heated until the sublimate which immediately forms upon the rings fixed above the pot begins to melt. The proportions of the two ingredients used vary largely, the best colours being probably produced when the mixture contains from one-third to one-fifth of sulphur; for the lighter colours, a smaller proportion of sulphur is employed. Orpiment made in this manner consists of a mechanical mixture of sulphide and oxide of arsenic. Orpiment is also employed as a dye, in the preparation of fireworks, and in some depilatories. The native sulphide is preferred to the artificial variety by artists and dyers, by reason of its richer colour; but it is a colour which in reality is hardly ever used now. Sometimes it is employed in water-colours, but as a pigment it is worthless. If it comes in contact with white lead it is decomposed in time, and a brown or black sulphide of lead is formed. While it endures it is a very brilliant colour. REALGAR.--Realgar or disulphide of arsenic, is a deep orange-red substance, soluble in water, and highly volatile and poisonous. It is found native in some volcanic districts, especially in the neighbourhood of Naples; but the commercial article is made by distilling, in earthenware retorts, arsenical pyrites, or a mixture of sulphur and arsenic, or of orpiment and sulphur, or of arsenious acid, sulphur, and charcoal, in the proper proportions; it has not the brilliant colour of the native mineral, and is much more poisonous.