SALTS OF COBALT. 569 



of cobalt, which must be perfectly free from iron or nickel, with 

 a solution of equally pure alum, precipitating the liquor by an 

 alkaline carbonate, washing the precipitate with care, drying and 

 igniting it strongly. It forms a beautiful blue pigment, known 

 as cobalt blue, which may be compared in purity of tint with 

 ultramarine. A compound of oxide of cobalt with oxide of 

 zinc may be prepared in a similar manner, which is a fine 

 green. 



Chloride of cobalt, Co Cl, is obtained by dissolving zaffre or 

 the oxide in hydrochloric acid. Its solution is of a pink red, 

 and affords hydrated crystals of the same colour; but when 

 highly concentrated, the solution assumes an intense blue colour, 

 and then affords blue crystals of chloride of cobalt, which are 

 anhydrous, (Proust.) The red solution is used as a sympathetic 

 ink : characters written with it on paper are colourless and 

 invisible, or nearly so, but when the paper is warmed by holding 

 it near a fire or against a stove, the writing becomes visible and 

 appears of a beautiful blue. By and by, as the salt absorbs 

 moisture, the colour again disappears, but may be reproduced 

 by the effect of heat. If the paper be exposed to too high a 

 temperature, the writing becomes black, and does not afterwards 

 disappear. The addition of a salt of nickel to the sympathetic 

 ink, gives a green instead of blue. 



The neutral carbonate of cobalt is unknown, oxide of cobalt, 

 like magnesia, being thrown down from its solutions, by alkaline 

 carbonates, as a carbonate with excess of oxide. The sub- 

 carbonate of cobalt is a pale red powder, which contains, ac- 

 cording to Setterberger, 2 eq. of carbonic acid, 5 eq. of oxide 

 of cobalt, and 4 eq. of water. 



Besides the sulphate of cobalt corresponding with green 

 vitriol, another salt was crystallized by Mitscherlich between 

 68 and 86, containing 6 eq. of water, Co O, SO 3 + 6HO, 

 isomorphous with a corresponding sulphate of magnesia. Sul- 

 phate of cobalt forms the usual double salts with sulphates of 

 potash and ammonia, containing 6 HO. 



Phosphate of cobalt, 2 Co O 2 , HO, PO 5 , is an insoluble 

 precipitate of a deep violet colour. When 2 parts of this phos- 

 phate, or 1 part of the arseniate of cobalt, is carefully mixed 

 with 16 parts of alumina and strongly ignited for a considerable 

 time, a beautiful blue pigment is obtained, having all the cha- 

 racters of ultramarine, which was discovered by Thenard. 



