INDIGO 905 



with a hard body show i slight bronze-like lustre. When carefully heated it may be 

 entirely volatilised, yielding a yellowish-red vapour, which condenses in the form of 

 long plum-coloured needles, having a slight metallic lustre. It dissolves in concentra- 

 ted sulphuric acid, forming a solution of a beautiful purple colour, which when diluted 

 with water yields no deposit and then imparts a fine purple colour to cotton, wool, and 

 silk. It is insoluble in water, but dissolves in boiling alcohol with a splendid purple 

 colour. It is insoluble in alkalis, but dissolves when exposed to the combined action 

 of alkalis and reducing agents, just as indigo-blue does, forming a solution from 

 which it is again precipitated on exposure to the oxygen of the atmosphere. This 

 solution dyes cotton purple. In most of its properties this body bears a striking re- 

 semblance to indigo-blue, and the composition of the two is identical. 



It has been doubted whether these various substances or impurities with which 

 indigo-blue is associated produce any effect in the dyeing process on cotton. In a 

 memoir by Schwarzenberg, to which a prize was awarded by the Soci6te Industriolle 

 de Mulhouse, the author arrives at the conclusion that neither indigo-gluten, indigo- 

 brown, nor indigo-red gives rise to any appreciable effect when added to an indigo vat 

 prepared with pure indigo-blue. Nevertheless differences are observable in dyeing with 

 different kinds of indigo, which can only be explained on the supposition that some- 

 thing besides indigo-blue takes part in the process. In the ordinary blue vat, made 

 with copperas and lime, any effect which might be produced in dyeing by the indigo- 

 brown is neutralised by the lime, which forms with it an insoluble compound. Indigo- 

 red, however, dissolves, as mentioned above, in contact with alkalis and reducing 

 agents, and the solution imparts a purple colour to cotton. In the ordinary indigo 

 vat its presence may be detected by precipitating a portion of the liquor, and treating 

 the precipitate with boiling alcohol, which then usually acquires a red colour. It is 

 possible, therefore, that a small part of the effect produced in dyeing with indigo may 

 be due to indigo-red. 



That portion of the indigo which remains after treatment with acid, alkali, and 

 alcohol consists essentially of indigo-blue, the true colouring matter of indigo, mixed, 

 however, with sand, earthy particles, and other impurities. In order to purify it, the 

 residue, while still moist, is to be mixed with lime, the quantity of which must amount 

 to twice the weight of the crude indigo, and which has been previously slaked with 

 water. The mixture is then put into a bottle capable of holding about 150 times 

 its volume of water, and the bottle is filled up with boiling water and shaken. A 

 quantity of finely-powdered protosulphate of iron, amounting to |rds of the weight of 

 the lime is then added, the bottle is closed with a stopper, well shaken, and left 

 to stand for several hours in a warm place. The mass gradually becomes green, and 

 the indigo-blue is then converted by the precipitated protoxide of iron into reduced 

 indigo, which dissolves in the excess of lime, forming a deep yellow solution. This 

 solution when clear is poured off from the deposit into a vessel containing a sufficient 

 quantity of dilute muriatic acid to supersaturate the whole of the lime. The reduced 

 indigo which is precipitated in greyish-white flocks, is agitated with water until it has 

 become blue, and the regenerated indigo-blue is collected on a filter and washed with 

 water, in order to remove the chloride of calcium and excess of muriatic acid. The 

 following method of obtaining pure indigo blue has been recommended by Fritzsche : 

 4 ozs. of crude indigo and the same weight of grape-sugar are put into a bottle capable 

 of holding 12 Ibs. of water; a solution of 6 ozs. of concentrated caustic soda-lye in 

 alcohol is then added, after which the bottle is filled with hot spirits of wine of 75 per 

 cent., and the whole is left to itself for some time. The liquid becomes at first wine-red, 

 then yellow, and on being filtered and left exposed to the air, deposits the indigo-blue 

 in small crystalline scales, which are to be filtered off and washed at first with alcohol, 

 and then with water. 



Pure indigo-blue has the following properties : Its colour is dark blue inclining to 

 purple. When rubbed with a hard body it assumes a bright coppery lustre. It has 

 neither taste nor smell, possesses neither acid nor basic properties, and belongs, as 

 regards its chemical affinities, to the class of indifferent substances. Its specific gravity 

 is 1*50. When heated in the open air it melts, boils, and burns with a smoky flame, 

 leaving a carbonaceous residue. But when it is heated in a vessel partially closed, or 

 in vacua, it. begins to evolve at a temperature of about 550 F. a violet-coloured 

 vapour, which condenses on the colder parts of the apparatus in the form of long 

 crystalline needles, which are blue by transmitted light, but exhibit by reflected light 

 a beautiful coppery lustre. These needles are unchanged indigo-blue. A great 

 portion of the indigo-blue is however decomposed during the heating process. Indigo- 

 blue is insoluble in water, alkalis, and dilute acids. Boiling alcohol and boiling oil 

 of turpentine dissolve a minute quantity of it, and deposit it again on cooling. Fixed 

 oils also dissolve a little of it at a heat exceeding that of boiling water, yielding blue 

 solutions, the colour of which, when the heat is further increased, changes, according 



