INDIGO.
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gradually added from 600 to 700 grms. of common nitric acid. As soon asthe blue colour has disappeared a large quantity of water is added, the liquidnext boiled, and filtered while still hot. From the filtrate impure isatinseparates in the shape of a brown amorphous mass, which being accompaniedby a rather large quantity of a semi-fluid resinous matter, is, in order to elimi-nate that substance, dissolved in caustic soda, and next very carefully neutralisedby hydrochloric acid, and immediately after filtered. By this process theresin is separated, and in order to obtain isatin from the deep orange filtrateit is only requisite to acidulate the liquor thoroughly, whereby the isatin isseparated as a crystalline deep orange-red body, sparingly soluble in cold, butbetter in hot, water; soluble in alcohol, ether, benzin, and consisting ofC 8 H 5 N 0 2 , that is is say, indigotin plus oxygen. Isatin is converted by nascenthydrogen at the ordinary temperature into isatyd, a white, very sparinglysoluble substance, which bears the same relation to isatin as white indigodoes to indigotin. When isatin and isatyd are heated together to 130° insealed tubes, along with either hydriodic acid or with biniodide of phosphorusand water, they are converted into a mixture of insoluble produdts, viz., isato-purpurin, a red substance, soluble in alcohol and ether; isatoflavin, a yellowmatter, insoluble in ether, but soluble in alcohol; and, lastly, a deep greensubstance, insoluble in alcohol and ether. These bodies are derivatives fromisatin by the elimination of oxygen, and may be viewed as polymers of in-digotin, with more or less oxygen in their composition. The reproduction ofindigotin from isatin by this readtion has not been successful, not a trace evenof that substance being found.
Caustic potassa converts isatin into isatic acid, while chlorine forms chlor-isatin and bichlorisatin, and bromine mono and bibromisatin. Isatin combineswith ammonia, forming, while water is simultaneously eliminated, imesatin,imasatin, isamic acid, isamid, and isatimid. Sulphurous acid combines withisatin, provided either potassa or ammonia be simultaneously present, yieldingcompounds known as isatosulphites. Isatyd combines with sulphurettedhydrogen to form sulphisatyd and bisulphisatyd, the sulphur taking the placeof a portion of the oxygen of the compound.
According to M. Laurent, the adtion of caustic potassa upon isatyd producesa red body, indin, which, although not fully identical with isatopurpurin, isvery similar to it. An alcoholic solution of caustic potassa converts indinand the sulphatisyds into hydrindin, a white or light yellow body, and intoflavindin, a deep yellow substance. Dr. Schiitzenberger has obtained similarprodudts by the reducing adtion of hydriodic acid upon isatin.
The substance known as indigo is chiefly a produdt of vegetable origin,but occurring also or formed in the lower as well as higher organised animals,including man, in some rare pathological cases of produdtion of a blue urineand blue pus. Indigo has undoubtedly been known in Asia from a very remoteperiod of antiquity, since there exist in very ancient records, written inthe Sanskrit language, descriptions of its mode of preparation, mainly notdifferent from the methods yet in use. The ancient Greeks and Romans havealso been acquainted with this drug, as proved by the testimony of Pliny,while the ancient Egyptians dyed linen and cotton fabrics with indigo, sincepieces of woven tissues dyed blue with this substance have been occasionallyfound among the wrappings of mummies.