A manual of elementary chemistry : theoretical and practical / by George Fownes.

  • Fownes, George, 1815-1849.
Date:
1873
Catalogue details

Licence: Public Domain Mark

Credit: A manual of elementary chemistry : theoretical and practical / by George Fownes. Source: Wellcome Collection.

    974/1062 (page 946)
    his researches on the formation of indigo-blue, which would over- step the limits of this elementary work, the reader is referred to Dr. Schunck's original papers.* The synthetical formation of indigo from acetophenone has heen already described (p. 843). Indigo comes into the market in the form of cubic cakes, which, when rubbed with a hard body, exhibit a copper-red appearance: its powder has deep-blue tint. The best indigo is so light as to float upon water. In addition to the blue colouring matter, or true indigo, it contains at least half its weight of various impurities, among which may be noticed a red resinous matter, the indigo-red of Berzelius: these may be extracted by boiling the powdered indigo in dilute acid, in alkali, and afterwards in alcohol. Pure indigo is quite insoluble in water, alcohol, oils, dilute acicls, and alkalis; it dissolves in about 15 parts of concentrated sul- phvuic acid, forming a deep-blue pasty mass, entirely soluble in water, and often used in dyeing: this is sulphinchjlic or sulphindi- gotic acid, a compound analogous to ethyl-sulphuric acid, capable of forming with alkaline bases blue salts, which, though easily soluble in pure water, are insoluble in saline solutions. If an insufficient quantity of sulphuric acid has been employed, or the digestion not continued long enough, a purple powder is left on diluting the acid mass, soluble in a large quantity of pure water. The Nordhausen acid answers far better for dissolving indigo than ordinary oil of vitriol. Indigo may, by cautious management, be volatilised: it forms a fine purple vapour, which condenses in brilliant copper-coloured - needles. The best method of subliming this substance is to mix it with plaster of Paris, make the whole into a paste with water, and spread it upon an iron plate. 1 part indigo and 2 parts plaster answer very well. This, when quite dry, is heated by a spirit-lamp: the volatilisation of the indigo is aided by the vapour of water disengaged from the gypsum, and the surface of the mass becomes covered with beautiful crystals of pure indigo, which may be easily removed by a thin spatula. At a higher temperature, charring and decomposition take place. In contact with deoxidising agents, and with an alkali, indigo suffers a very curious change: it becomes soluble and nearly colourless, perhapsreturning to the same state in which, it existed in the plant. It is on this principle that the dyer prepares his indigo-vat: 5 parts of powdered indigo, 10 parts of green vitriol, 15 parts of slaked lime, and 60 parts of water, are agitated together in a close vessel, and then left to stand. The ferrous hydrate, in conjunction with the excess of lime, reduces the indigo to the soluble state; a yellowish liquid is produced, from which acids precipitate * Memoirs of the Literary and Philosophical Society of Manchester, vol. xii. 177; xiv. 181, 239 ; also Philosophical Magazine [31 x. 73 : \ v. 99; [4], xv. 29, 117.