Page:The American Cyclopædia (1879) Volume IX.djvu/263

 INDIGO INDIUM 251 tious addition of protochloride of tin ; the in- soluble portion being then separated, bichro- mate of potash recovers and throws down the indigo blue, which when washed with hydro- chloric acid is collected and weighed. Others adopt the plan of first taking up impurities by a succession of appropriate solvents ; these impurities are chiefly resinous and gummy matters. The pure indigo has also been ob- tained by another process dependent on its property of volatilizing at the temperature of about 550 F., and condensing in needle-shaped and prismatic crystals. The operation is hast- ened by mixing the indigo with water and twice its weight of plaster of Paris to a paste, which is spread on an iron plate. Heated over a spirit lamp, the steam and vapor of indigo separate together, and the latter collects in beautiful velvety crystals, upon the surface of the mass. The pure substance melts nearly at the temperature at which -it sublimes, and is also charred and decomposed at about the same. It also ignites and burns with a bright flame, giving off much smoke. The crystals have a beautiful and intense copper color, and when in thin plates they present by transmitted light a splendid blue. Their composition is represented by the formula CnHioNjOj. The substance resists in a remarkable degree the action of the ordinary solvents (sulphuric acid excepted), unless it be first deoxidized, when it readily dissolves in alkalies. The conversion into colorless indigo, though called deoxidation, appears to be rather an accession of two atoms of hydrogen, which, according to Liebig, unite with one of the oxygen present, giving to the body the composition of a hydrate, having the formula Ci.HioNjO + HaO or CmHuN^Oj ; the indigo blue in this case being an oxide of the same body, CuHioNjO. The facility with which the change is effected, and the readiness with which the indigo regains its blue color and insolubility by exposure to the air, admi- rably adapt the substance for use as a dye. It is applied in the solvent state to the fabric steeped in the liquid ; and when the cloth is exposed to the air, the insoluble substance is developed with its characteristic color and fixed in the fibres. Sulphuric acid dissolves indigo blue without changing its color to red, the usual action of acids upon vegetable blues ; and when the substance is digested for three days with 15 parts of concentrated sulphuric acid, a deep blue pasty mass is obtained, which dissolves completely in water, and under the name of sulphindylic acid, or more properly hyposulphoindigotic acid, is often used in dye- ing, and also in the manufacture of the blue inks. Many other beautiful and highly inter- esting bodies result from this chemical change, and still more from the oxidation of indigo blue; and still another series from its treat- ment with the alkalies. These have received much attention from eminent chemists, and are particularly treated in Dumas's Traite de cjiimie appliquee aux arts, vol. viii., in Brando's " Manual of Chemistry," and in Muspratt's " Chemistry." Indigo has been somewhat used in medicine, but is not at present recognized as a remedy of value. It sometimes produces nausea and vomiting. It colors the stools bluish black, and also passes into the urine. Indican has occasionally been found in the urine when no indigo has been taken. The dose is from 30 to 120 grains. INDIGO BIRD (cyanospiza cyanea, Baird), a North American finch, of a blue color, tinged with ultramarine on the head, throat, and mid- dle of breast, and elsewhere with verdigris green ; lores and angle of chin velvet black, and wing feathers brown edged with bluish brown. The length is about 5$ in., and the extent of wings 7i ; the female is smaller, and yellowish brown with the wings darker. It is found in the eastern United States, as far west as the Missouri, and south to Guatemala ; it ar- Indigo Bird (Cyanospiza cyanea).!. Male. 2. Female. rives in the southern states from Mexico and Central America about the middle of April with the painted finch (G. ciris), and like this is caught in traps for sale. It prefers open places on the edges of woods ; perched on the top of a high tree, it delights to sing its clear and sweetly modulated strain, consisting of eight or ten notes. Though less handsome than the painted finch, its shape is elegant and compact, and its manners very lively, so that it is in request as a cage bird. The nest is made among the rankest grass, and the eggs, four to six, are blue, with one or two purple spots on the larger end. They migrate southward in the autumn. The food is small seeds and insects. INDIUM, a rare metal discovered in 1863 by Professors Eeich and Kichter of Freiberg, Saxony, by means of spectrum analysis, and so named from two indigo-colored lines in the more refrangible part of the spectrum. It was originally detected in the zinc blende of Frei- berg, and has since been found by Bottger in the flue dust of the zinc furnaces at Goslar, by