(2) A series of compounds described below. [_a_] A thorough mixture of 2 parts sulphur and 1 part dry carbonate of soda is gradually heated in a covered crucible to redness or till fused; a mixture of silicate of soda and aluminate of soda is then sprinkled in, and the heat is continued for an hour; the little free sulphur present may be washed out by water. [_b_] An intimate mixture of 37 parts china-clay, 15 parts sulphate of soda, 22 parts carbonate of soda, 18 parts sulphur, and 8 parts charcoal, is heated in large crucibles for 24-30 hours; the mass is re-heated in cast-iron boxes at a moderate temperature till the desired tint appears, and is finally pulverised, washed, and dried. [_c_] Gently fuse 1075 oz. crystallised carbonate of soda in its water of crystallisation; shake in 5 oz. finely-pulverised orpiment, and, when partly decomposed, as much gelatinous alumina hydrate as contains 7 oz. anhydrous alumina; add 100 oz. finely-sifted clay, and 221 oz. flowers of sulphur; place the whole in a covered crucible, and heat gently till the water is driven off, then to redness, so that the ingredients sinter together without fusing; the mass is then cooled, finely pulverised, suspended in river-water, and filtered. The product is heated in a covered dish to dull redness for 1-2 hours, with occasional stirring. Colourless or brownish patches may occur, and must be removed. SAXON BLUE.--Following is a recipe for the preparation of this pigment, which possesses limited importance. Dissolve 8 lb. alum and 1 lb. green copperas in 16 gallons of water. Add separate solutions of pearlash and yellow prussiate till precipitate ceases to go down. Collect the precipitate when it has completely settled; wash thoroughly, and dry. SOLUBLE BLUE.--This term is applied to a variety of Prussian blue which, while possessing no difference in the matter of chemical composition, yet has the distinctive feature of being soluble in water, which the other varieties are not. It no longer enjoys the popularity it once had as a dye, on account of the severe competition of the coal-tar colours. Below are some of the most satisfactory formulæ for its preparation. (1) Mix 10 lb. of Prussian blue thoroughly in about 10 gallons of cold water. Then add 5 lb. of yellow prussiate and let the whole mass boil steadily for several hours. Strain off the liquor and well wash the precipitate on a filter. Finally dry for use. (2) Dissolve about 1 cwt. of red prussiate in water and make the solution hot. Prepare another solution of about 73 lb. of green copperas in hot water. Mix the two solutions together and boil them for about a couple of hours. Allow the solid matters to settle out, then put them on a filter and wash with clean water until a blue coloration manifests itself in the drainings. The blue residue is then dried as usual.