=Mercuric Iodide.=—When corrosive sublimate solution is precipitated with exactly the necessary quantity of potassium iodide, mercuric iodide is obtained as a scarlet precipitate which surpasses in beauty even the best samples of vermilion. Unfortunately, this substance cannot be used as an artists’ colour. Exposure to light soon turns it brown and finally black. It appears to be unaltered in the dark; the author possesses a sample which has been so kept for 30 years without losing its shade in any way. CHAPTER XVIII. ANTIMONY VERMILION. Antimony vermilion is a red pigment which will bear comparison in fineness of shade with mercury vermilion, over which it has the advantage of cheapness. In composition it is antimony trisulphide, Sb₂S₃. This compound is obtained by precipitating a solution of antimony trichloride with sulphuretted hydrogen. However, the precipitate, which is a very fine red whilst wet, loses its colour in drying, and the product is almost worthless as a pigment. In another way it can be obtained in such a condition that it loses nothing of its beauty in drying, but retains its brilliance. Böttger gives the following process: a solution of antimony trichloride is mixed with a solution of sodium hyposulphite (thiosulphate) and the liquid heated so long as a precipitate forms, which is then washed on a filter with water containing acetic acid. If pure water were used for washing, the antimony chloride still present would be decomposed, forming the white oxychloride, which would detract from the shade of the antimony vermilion. In this process particular regard is to be paid to the use of exact quantities of materials. The finest product is obtained when 2 parts of a solution of antimony trichloride, which has exactly the specific gravity 1·35, are mixed with a solution of 3 parts of sodium hyposulphite in 6 parts of water. According to R. Wagner, antimony vermilion is obtained by dissolving 4 parts of tartar emetic and 3 parts of tartaric acid in 18 parts of water, heating to 60° C., mixing with a solution of sodium hyposulphite (thiosulphate) and heating to 90° C. The precipitate is then carefully washed and dried. Pure antimony vermilion closely approaches, as we have said, ordinary vermilion in shade, and for a sulphur compound shows a remarkable resistance towards chemical reagents. By dilute acids, ammonia and alkaline carbonates, it is attacked only on long continued contact, but it is easily decomposed by very dilute hydrochloric acid and by caustic alkalis. A mixture with white lead keeps for a long time, but there can be no question of the permanence of such a mixture, in consequence of the oft-repeated properties of lead pigments. Antimony vermilion is well adapted for oil painting. When ground with oil it exhibits a red of a brilliance in no way inferior to that of genuine vermilion. It may also be used as a water colour, but is not adapted for fresco work, since it is quickly decomposed by lime.