INDJGOTIN. 439

blue, and the form of the crystals is a right rhomboidal prism, terminating intwo strongly inclined planes.

Amorphous indigotin, as obtained by the oxidation of solutions of whiteindigo in contadt with air, is deep blue, with a purplish hue, and assumes byrubbing a coppery lustre.

Indigotin volatilises at 290°, but the volatilisation is usually accompanied bypartial decomposition ; and only when the pure material is operated upon, andthe volatilisation aided by some inert gas, decomposition is prevented.

Sublimed indigotin may, according to Dumas, be readily obtained by placinga thin layer of indigo in a clean frying-pan, and using another frying-pan ascover, the lower pan containing the indigo being cautiously heated, preferablyon a sand-bath. Indigotin and indigo produce, when strewed over red-hotcoals, violet vapours, not unlike those arising from heated iodine, and simul-taneously a peculiar aromatic odour is given off.

Indigotin is insoluble in water, alcohol, ether, fatty and essential oils, anddilute acids and alkalies, either hot or cold. Creosote (Reichenbach's) andphenic acid and aniline dissolve a small quantity of indigotin at a boilingheat, but, on cooling, it is re-deposited. When finely pulverised indigotin ismixed with anhydrous acetic acid, and a single drop of concentrated sul-phuric acid is added, there is obtained a beautiful deep blue liquid, from whichthe indigotin is separated unaltered on the addition of water. If the originalacetic acid solution is applied to any woven tissue, and immediately after-wards washed in water, the tissue is dyed blue. It should be observed thatthis process is the only manner hitherto known in which indigotin, withoutbeing reduced, is reproduced in its primitive state. Indigotin is a thoroughlyneutral substance, void of taste and smell ; its sp. gr. is r - 35. When decom-posed by dry distillation indigotin yields, among other products, aniline. Byoxidising agents, such as concentrated solution of chromic acid and chlorine,indigotin is converted, when water is simultaneously present, into a new sub-stance, which differs from indigotin only by having one atom of oxygen more inits composition, and which is named isatin. It is usually obtained from indigotinby the adtion of moderately diluted nitric acid. Strong and hot nitric acidconverts indigotin into indigotic or nitrosalycilic and picric acids : in either ofthese reactions some carbon is eliminated, in the shape of carbonic or oxalicacid. The most interesting, and pradtically most valuable, property of indigotinis its great power of fixing hydrogen, and becoming converted into a colourlessbody, soluble in alkalies or alkaline earths, and capable of reproducing indigotinby simple exposure to the oxidising adtion of the air. This phenomenon ofhydrogenisation, improperly called redudtion, requires the hydrogen to be inthe nascent state, and it is generally considered that this readtion can only takeplace in the presence of alkalies or alkaline earths only, but Schlumberger hasobserved this phenomenon taking place in an acid liquid, the conversion ofindigotin into white indigo being quite complete.

It is well known that the freshly-prepared solution of zinc in sulphurousacid decolourises energetically the solution of sulphate of indigo, as well asother colouring matters. When a concentrated solution of sulphurous acidis poured over granulated zinc, the liquid becomes first yellow, and rapidlyacquires this decolourising property, there being formed a mixture of sulphiteand hyposulphite of zinc: none of the substances present in this mixture