Wagner found that an indispensable condition was to have a protoxide of cobalt as free as possible from foreign metals, with which object he practised the following method:--Cobalt oxide is dissolved in three equivalents of hydrochloric acid, and the solution is evaporated to dryness; the residue is dissolved in six equivalents of water, and through the solution is passed a current of sulphuretted hydrogen gas, so long as any precipitate is formed. This precipitate consists of sulphides of the foreign metals. The clear solution is siphoned off, evaporated to dryness, and the residue is dissolved in water. As required, this solution is treated with carbonate of soda, and the precipitate, washed, and while still wet, is mixed with zinc white. The reddish mass produced in this way is dried and calcined. The best tone is attained by combining 9 to 10 parts of zinc oxide with 1 to 1½ parts of cobalt protoxide. Louyet has shown that if the cobaltic solution be precipitated by the phosphate or the arseniate of potash, the corresponding salt of cobalt thus produced possesses the property of imparting a green colour to zinc white at a much lower temperature than is required in the case of ordinary protoxide of cobalt: moreover, the pigment gains in body, and the colour gains in purity and brightness. If a small quantity of arsenious acid is added to the ordinary mixture before calcination, the calcined mass will assume a remarkably bright green colour; and its structure being loosened by the disengagement of fumes of arsenious acid, it will be easy to grind. According to Barruel and Leclaire’s method, 1 lb. of pure dry sulphate of cobalt, dissolved in hot water, is mixed with 5 lb. of zinc oxide. The mixture is dried, and calcined for three hours at a clear red heat in a muffle; when cooled, it is thrown into water, washed, and dried. The composition of cobalt green has been shown by Wagner to vary considerably, as is to be expected from the methods of its preparation. The proportion of zinc oxide ranges from 71½ to 88 per cent., and the cobalt protoxide from 11½ to 19 per cent.; in addition, there will be fluctuating percentages of phosphoric acid, soda, oxide of iron, &c., according to the process followed. With the single exception of its costliness, cobalt green possesses advantages over most other green pigments. It has a bright colour, sometimes inclining to a yellowish tint, or, when phosphates are used in its preparation, leaning to a blue shade. But it is always permanent, not only under the influence of air and light, but also in the presence of alkalies and any but concentrated acids; thus it may safely be compounded with other pigments.