204
DYEING AND CALICO PRINTING.
small portion of the liquid is taken and filtered over sand. When the filtrateruns through a fine green, without any mixture of blue or violet, the additionof carbonate of soda ceases, i£ to i| kilos, of crystals being generally suffi-cient. The whole liquid, when cold, is now filtered over sand, to remove blueand violet matters. The filtrate is precipitated by a cold saturated solution ofperfectly pure picric acid. For the proportions taken, 3*46 kilos, of picric acidwill be required. The picrate formed is but sparingly soluble in water, and isconsequently thrown upon a common filter and slightly washed. The preci-pitate is a very fine paste. It is allowed to drain till sufficiently concentrated.The dyer is obliged to make use of a small proportion of alcohol in dissolvingthis green.
Soluble Green.
In preparing this colour we substitute for the picric acid the sulphate, chloride,or acetate of zinc; there are thus obtained double salts of zinc, and of the
Fig. 20.
1 i 3.FEE7
f
a b. Boiler and lid. c. Bath, containing water heated by steam circulating in theworm, d. An oscillatory movement on the trunnions, <?, is given to the boiler by
the excentric,/, the crank, g, and the handle, h. This oscillatory movement may bereplaced by an internal agitation of the contents by means of the pulley, j, theshaft, k, and the paddles, m.
dimethyl-hydriodate of trimethyl-rosaniline. These compounds are soluble inwater, and of a less yellow tone than the picrate. They are sold in the drystate.
The yield of the process should be about 60 per cent of green and 40 ofviolet, which can be transformed into green as follows :—The violet is wellwashed with water, dried, and powdered. It is then dissolved in pure alcohol(free from methylic spirit), and its base is liberated by the addition ofcaustic soda. It is then put in a suitable apparatus, provided with an agitatorand a cohobator. An equivalent weight of methylic iodide is added, and theapparatus is carefully heated in the water-bath, so as not to exceed 48° C. Assoon as the conversion is over, which is very rapid, the whole is quickly cooled,acetic acid is added, and the contents are poured into a large wooden vatcontaining hot water. The green is separated from residual traces of violet