914 NEUTRAL COLOURING MATTERS. 



for several hours, and kept in a warm place till the supernatant 

 liquor acquires a yellow colour. The protoxide of iron precipi- 

 tated by the lime becomes peroxide, taking oxygen from the 

 blue indigo, which in this altered state forms a compound with 

 the lime, soluble in water. When the yellow solution of this 

 compound is poured out or exposed to air, it rapidly becomes 

 blue from the absorption of oxygen, the indigo loses its solubility 

 and precipitates. In the usual process of dyeing, the indigo is 

 fixed by dipping the yarn or cloth in the same solution of 

 indigo, and then exposing it to air ; the indigo thus penetrates 

 into the cloth in a soluble state, and is rendered insoluble 

 afterwards by oxidation within its substance, so that it cannot 

 afterwards be washed out. The sediment in the bottle yields 

 more soluble indigo, when agitated again with pure water 

 slightly heated, or with lime-water. The yellow solutions are 

 mixed together and freely exposed to air, with the addition of 

 a little hydrochloric acid to dissolve the salts of lime, and the 

 blue indigo which precipitates is collected, washed upon a filter 

 and dried. It is afterwards washed with boiling alcohol by 

 M. Dumas to take up the indigo red of Berzelius. 



The purified indigo, when dry, is of a deep blue, with a shade 

 of violet, and when rubbed, exhibits a strong copper red colour 

 and metallic lustre. It is tasteless, inodorous, insoluble in 

 water, alcohol, ether, and not affected by alkalies or diluted 

 acids. When heated to about 550, it fuses (Crum), and at the 

 same time emits a beautiful purple red vapour and sublimes, 

 condensing in small copper-coloured prisms, but it is always 

 partially decomposed at the same time. To observe the beau- 

 tiful appearance of sublimed indigo, ten or twenty grains of 

 good indigo in powder may be put upon a pretty thick sheet of 

 iron or copper, pressed flat and then covered by the lid of a 

 platinum or porcelain crucible, two or three inches in diameter, 

 while the plate is heated sharply by placing it over a lamp or 

 ch offer. On raising the cover, after the plate is cool, the charred 

 mass will be found entirely covered by copper-coloured crystals. 

 Their density is 1.35. Blue indigo was carefully analysed by 

 Mr. Crum,* and repeatedly by M. Dumas, whose results con- 

 firm the original determination of Mr. Crum.f 



* Thomson's Annals of Philosophy, second series, v. 82. 



t Du-:iias, Ann. de Chirn. et de Phys. t. 73, p. 269 ; and 3 se>. t. 2, p. 204. 



