=Thallium Pigments.=—Thallium is a metal which exhibits certain similarities to lead. By precipitating a solution of a thallium salt with potassium chromate or bichromate, according to the proportion between the quantities of the two salts, precipitates are obtained of yellow, orange or deep red colour, or, after fusion, brown. By the addition of a mixture of potassium chromate and ferricyanide to a mixture of a thallium salt and ferrous sulphate an olive green pigment is obtained. On account of the rarity of thallium compounds, technical employment is out of the question, and the sensitiveness of thallium pigments towards sulphuretted hydrogen prevents their use for artistic purposes. CHAPTER XV. MOSAIC GOLD. Mosaic gold consists of tin disulphide, SnS₂, fine scales of a golden yellow colour, which sublime undecomposed at a fairly high temperature and withstand the action of chemical reagents. It has a peculiar greasy nature, and can be easily ground. It is, therefore, much used for bronzing picture frames, as a pigment for painters and for wall papers. Tin disulphide can be prepared either in the wet or the dry way; in the wet way, by the action of sulphuretted hydrogen on a solution of tin tetrachloride. The yellow precipitate so obtained has no handsome colour. A far finer product is obtained in the dry way. The process is often regarded as accompanied by particular difficulties, but in reality it is quite simple. It is only necessary in preparing this pigment to take care not to raise the temperature above a certain point, otherwise a great portion of the tin disulphide will be decomposed into sulphur and tin monosulphide. To prevent the temperature from rising too high, an addition of ammonium chloride is made. This salt is volatile at a certain temperature; heat which would otherwise raise the temperature above this point is used in volatilising the ammonium chloride. With a little care it is easy to interrupt the operation before all the ammonium chloride has been driven off. The mosaic gold then obtained has a real golden glitter. If the temperature rises too high, grey tin monosulphide is formed, which naturally considerably diminishes the brilliancy of the product. Ammonium chloride may be replaced by mercury or mercury compounds, which are volatile at a temperature below that at which mosaic gold is decomposed. When mercury compounds are used, both on account of their cost and poisonous nature the heating must be conducted in glass retorts in order to recover the mercury. This operation requires great care if loss due to the breakage of the glass vessels at the high temperature is to be avoided. The process in which metallic mercury is used gives the finest product of all, and is to be recommended when a pigment is to be prepared which shall as nearly as possible approximate to the appearance of gold.