COBALT BLUES.

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French blue is obtained by the following process:—The tissue is treatedtwice with a solution of stannate of soda at 3 0 B.; then passed twice througha bath, composed of 12 parts of a solution of nitrate of peroxide of iron at45 0 B., 1 part of tin-salt (crystallised protochloride), and i£ parts of sulphuricacid. After rinsing through water the tissue is immersed in a bath, composedof 2 parts of yellow prussiate and 1 part of sulphuric acid and sufficientwater.

Havraneck green is composed as follows :—1 part of red prussiate, 4 partsof yellow prussiate, 2 parts of chrome alum, 9 parts of prussiate of tin in pulp,1 part of tartaric acid, 24 parts of water thickened with starch. The colouris developed by steaming.

Cobalt Blues.

The substance known as casruleum is a stannate of cobalt mixed with somesulphate of lime, the latter being unessential. The formula of this compoundis 3(Sn0 2 ,CoO) + Sn 0 2 . The pigment is not affeCted by atmospheric influence,light, or heat; it is neither affected by caustic alkalies or acetic acid, butis dissolved by mineral acids. It is blue also in artificial light. A factitiouscasruleum is obtained by mixing ultramarine with Naples yellow and white-lead. Neither this cseruleum nor smalt (a cobalto-silicate), now only used forporcelain staining, nor Thenard blue (oxide of cobalt and alumina, or phos-phate of alumina with oxide of cobalt), a very beautiful blue, are now used aspigment colours for calico-printing.

None of the blue-coloured compounds of molybdenum and tungsten are atpresent used, either as dyes or as pigment colours, but the introduction ofthese colours may be expected within a few years.