Dr. Schunck on the Formation of Indigo-blue. 43 



liquid was then strained as quickly as possible through canvas, 

 and left to stand exposed to the air in a shallow vessel. The 

 surface of the liquid soon became covered with a purple pellicle^ 

 which was carefully skimmed off and was succeeded by another, 

 which was in its turn removed, the process being repeated as 

 long as anything formed on the surface. The purple matter 

 was then dissolved a second time in an alkaline solution of prot- 

 oxide of tin, and the solution was again left exposed to the 

 atmosphere. The pellicle which was formed by the action of 

 the oxygen was removed this time by means of blotting-paper, 

 to which it adhered without much of the liquid underneath being 

 removed with it. The substance was separated from the paper 

 by agitation in water, collected on a filter, ti-eated with boiling 

 caustic soda to dissolve a little adhering fatty matter, filtered off 

 again, washed with acid, then with water, and lastly dissolved in 

 boiling alcohol. The alcohol acquired a splendid purple colour, 

 and on cooling deposited a quantity of crystalline needles, con- 

 sisting, as I believe, of indirubine in a state of purity. When 

 thus prepared it is found to have the following properties. 



It crystallizes from its alcoholic solution in small needles, 

 forming when dry a silky mass of a colour between purple and 

 chocolate, which, on being rubbed with a hard body, shows a 

 sligbt metallic lustre resembling that of bronze. When heated 

 on platinum foil it emits red vapours, then melts and biu'ns with 

 a yellow smoky flame, leaving some charcoal. When carefully 

 heated between two watch-glasses it gives a yellowish-red vapour, 

 resembling that of bromine, which condenses on the upper glass 

 in the form of beautiful long crystalline needles. These needles 

 are plum- or garnet-coloured ; they possess a somewhat metallic 

 lustre, which is however much inferior to that of su.blimed indigo- 

 blue, and seem to consist simply of the original substance, which 

 has been volatilized without change. When the process of sub- 

 limation is carefully conducted, only a trace of carbonaceous 

 residue is left. It dissolves comj)letely in concentrated sulphuric 

 acid in the cold, forming a solution of a beautiful purple colour. 

 This solution when heated does not become black, but on the 

 contrary rather paler, and evolves only a trace of sulphurous acid. 

 When mixed with water it gives no precipitate and retains its 

 fine purple colour, which does not disajjpear or become weakened 

 when the acid is neutralized with carbonate of soda, but soon 

 vanishes entirely when an excess of caustic soda or ammonia is 

 added. The sohjtion in sulphuric acid after dilution with water 

 imparts a fine purple colour to cotton, wool, and silk. When 

 treated with nitric acid of ordinary strength, indirubine begins 

 to dissolve oven in the cold, and to a greater extent on the 

 application of heat, forming a purple solution, which ou being 



