844 I N D I N D 2 90- 300 C., giving off violet vapours which condense into right rhomboidal prisms possessing a brilliant coppery lustre. By destructive distillation, indigotin yields, among other products, aniline, a circumstance to which that now well-known body owes its name (from the Sanskrit nili through the Portuguese anil, indigo). Treated with oxidizing agents, indigotin takes up oxygen, and is con verted into isatin, thus : CH 10 N a O a +20-C M H 10 N a 4 ; Indigotin. Isutin. and by further oxidation nitre-salicylic acid and picric acid are evolved. The most valuable character, however, of indigotin is found in its behaviour under the influence of hydrogenizing or reducing agents. In the presence of nascent hydrogen indigotin absorbs that element and is converted into white indigo, a colourless body which is readily soluble in alkaline or earthy alkaline solutions, and by simple exposure to the air re-oxidizes and reverts to its original blue condition indigotin. The reduction to white indigo is thus formulated : C 1(i H 10 N A + 2H = deHuNA. Advantage is taken of these properties in dyeing with indigo as detailed under DYEING, vol vii. pp. 576-7. See also CALICO-PRINTING, vol. iv. pp. 689-90. Indigo when dissolved in strong sulphuric acid, forms with it two acid compounds, both of which have limited industrial applications. These are (1) sulphindigotic acid, C 16 H 8 N O 4 (S0 3 H).,, known also as sulphate of indigo or soluble blue indigo, and (2) sulphophcenicic acid, sul- phopurpuric acid, or indigo purple, C 1(5 H 9 N 0.,(S0 3 H). These bodies are formed together in the sulphuric acid solution of indigo ; but, as the second is insoluble in weak acids, it precipitates when the solution in which it is formed is largely diluted with water. Both these acids are soluble in water. The first was formerly used in dyeing Saxon-blue on wool and silk, a style now little known ; and the sodium salt of the second is known as red indigo carmine. The synthetical preparation of indigo is a subject which has long occupied the attention of chemists, as obviously any means by which the substance might be artificially obtained on a commercial scale could not fail to be of great industrial value. The numerous efforts made in this direction appear at last (1881) to be crowned with success; and there is now little doubt that artificial indigo will soon become a commercial product. It is to Professor Adolf Baeyer of Munich that the measure of success already attained in manufacturing indigo is due. For many years he has patiently investigated the molecular constitution of indigotin and its derivatives. From isatin, prepared by the oxidation of indigotin, Baeyer and Knob produced successively di-oxindol, C 1G H U N 2 O 4 , oxindol, C 16 H 14 N 2 O 2 , and inclol, C 10 H 14 N" 2 . Baeyer at a later period, with the assistance of Emmerling, succeeded in producing indol from cinnatnic acid, and as that body can be prepared from coal- tar a new connecting chain was established between indigo at one extreme and coal-tar at the other, meeting in indol just as at a much earlier date they had similarly met in aniline. The task remained of reconverting these deriva tives of indigotin into that body, and towards that, in 1870, Baeyer and EmmerHng, by heating isatin with phosphorus trichloride, acetyl chloride, and phosphorus, succeeded in obtaining a mixture of indigotin and indigo- rubin. In 1878 the further steps necessary to complete the cycle were accomplished by Baeyer, when from phenyl- acetic acid he prepared oxindol. Acting on oxindol by nitrous acid he produced nitrosoxindol, which in its turn, "by treatment with nascent hydrogen, was transformed into amidoxindol, a body which on oxidation yielded isatin. Thus the series of transformations was complete ; but they were effected by a process so roundabout and elaborate as to preclude all hope of any commercial issue from the method. Quite recently Baeyer, coining back to the use of cinnamic acid, has devised the much simpler and more direct process which now promises to become, and indeed already is in operation as, a method for the com mercial preparation of indigo. By treating cinnamic acid w T ith nitric acid, ortho-nitro-cinnamic acid is prepared, which on exposure to bromine vapour readily combines with that body, forming ortho-nitro-dibrom-hydro-cinnamic acid. This substance when treated with caustic alkali is converted by the loss of the bromine into ortho-nitro- phenyl-propiolic acid, which, lastly, when heated in an alkaline solution of grape sugar develops into indigotin. The steps in the process are therefore represented thus : (1) C 6 H B C 2 H 2 COOH + N0 2 HO = C 6 H 4 (N0 2 )C 2 H 2 COOH + H 2 . Cinnamic acid. Nitiic acid. Nitro-cinnaiiiic acid. Water. (2) C 6 H 4 (NO a )C 2 Nitro-cinnamic acid. C 6 H 4 (NO a )C s H 2 Br 2 COOH . Bromine. Nitro-dibrom-cinnamic acid. Caustic soda. Nitro-propiolic acid. (4) 2C 9 H S N0 4 + 2H, = C J6 H 10 N 2 2 + 2C0 2 + 2H 2 . Nitro-propiolic Indigotin. acid. The nitro-propiolic acid is now being manufactured by the Badische Anilinfabrik as a material for indigo printing. The acid has simply to be printed on the cloth with a thickening containing grape sugar and alkali, and, by steaming, indigo is developed in the fibre. This reaction is in itself a matter of no small importance, seeing that the printing of indigo direct is an extremely troublesome operation. Hitherto indigo in mass has not been pro duced, but there can be little doubt that the remaining difficulties, among which is the present expensiveness of cinnamic acid, will soon be overcome, and that artificial indigo will take its place among ordinary chemical manu factures. (j. PA.) INDIUM, a metal discovered with the aid of the spectroscope in 1863 by Reich and Richter when testing certain specimens of Freiberg zinc-blende for thallium. Instead of the brilliant green line characteristic of this latter metal, they observed an intense indigo-blue line occupying a position different from that of any known line, and w^ere thus at once led to suspect the presence of a previously unknown element. The name indium was chosen for the metal, when they succeeded in isolating it, on account of this circumstance. It has since been detected in blendes from various sources, but always in extremely minute amount, and still remains one of the rarest of the elements. Indium is best prepared from crude zinc made from indium-containing blendes. As it is less positive than zinc, if the crude zinc is treated with insufficient hydro chloric acid to dissolve it completely, a residue is obtained containing all the indium together with several other metals also present in small quantity in the zinc. The properties of indium have already been partially described (vol. v. p. 533). Its flame spectrum exhibits, besides the indigo-blue line (w. 1. 4509), a violet line of w. 1. 4101. Lockyer has stated (Ro>/al Society Proceedings, 1878, xxviii. p. 177) that the strongest line in photographs of the spectrum of indium in the electric arc is, as already recorded by Thalen, the h line of hydrogen, the hydrogen line near G being, however, absent. He argues that this is not to be explained by the supposition that the indium contains occluded hydrogen, since none of the hydrogen lines become impressed on the plate when palladium-hydrogen is volatilized in the arc. Indium is commonly regarded as closely allied to aluminium, on account of the general resemblance of corresponding compounds of the two metals, and especially on account of
the existence of an indium alura isomorphous with ordinary
