136 TEXTILE FABRICS. [V. 



solution of carbonate of soda (3-4 oz.) until all is precipitated. 

 The precipitate is filtered, washed, pressed in linen, and dried 

 in the shade. It has a rich carmine color, and rubbed on glass 

 with a good oil-varnish, appears as transparent as red glass. 

 Its solution in strong acetic acid gives a beautiful and durable 

 red ink, which does not mould and requires no gum. It may 

 be diluted at pleasure. 



For the properties of a new dye of somewhat analogous 

 character to madder, we refer to the Lond. Journ. xxxvi. 335. 



Red Coloring-matter of Rliuharh. — The erythrose of Garot 

 (Journ. de Pharm. et de Chim. 1850) is extracted from rhubarb 

 by acting on 1 pt. with 4 pts. of nitric acid. The residue, 

 remaining untouched, is the coloring-matter, and varies in 

 different rhubarbs from 8 to 20 per cent. It is orange-red, 

 soluble in alcohol and ether, and forms with the alkalies red 

 compounds of eminent tinctorial power. It is said to give a 

 dye of much greater brilliancy and stability than that from 

 cochineal. 



Cochineal. — Warren de la Rue (Chem. Soc. Trans. 1847) 

 thus obtains the coloring-matter. 3ib powdered cochineal are 

 boiled for 20 minutes in 60 litres distilled water, strained, and 

 poured off clear in J an hour. It is then precipitated by a 

 solution of acetate of lead previously acidulated by acetic 

 acid (6 pts. acetate, 1 pt. acid), the precipitate washed with 

 boiling water, dried in warm air, and powdered. This yields 

 17 oz. crude carmine-oxide of lead. The cake is suspended in 

 water, sulphuretted hydrogen passed through, the deep-red 

 solution evaporated in a water-bath, and lastly dried in vacuo. 



The crude carminic acid contains phosphoric acid, and to 

 purify it, it is dissolved in boiling absolute alcohol, and digested 

 for some hours with carminate of lead. Ether is added to the 

 solution, which precipitates a little nitrogenous matter, and 

 the filtrate evaporated in a retort, and finally dried in vacuo. 

 It is the pure cochineal-red or carminic acid. 



This substance is a purplish-brown pulverizable mass, 

 transparent under the microscope, of a beautiful red color 

 when finely divided, decomposed by chlorine, bromine, iodine. 



