Dr. Schunck on Colouring Mailers. 22 1 



sulphuric acid with a dark brown colour, and is re- precipitated by 

 water. Boiling dilute nitric acid decomposes it with an evolution 

 of nitrous acid, and changes it into a yellowish-red (locculent sub- 

 stance. Concentrated nitric acid on boiling decomposes and dis- 

 solves it entirely. It dissolves in caustic and carbonated alkalies 

 with a dark brown colour, and is re-precipitated by acids in light 

 brown flocks. The ammoniacal solution gives brown precipitates 

 with the chlorides of barium and calcium. The dark green powder 

 which is produced by the action of sulphuric and muriatic acid on 

 xanthine, has the following properties: — When dry it has a dark 

 olive colour. It burns with a flame and a smell like burning wood, 

 leaving a large quantity of charcoal, which however burns away 

 without any fixed residue. It is decomposed by boiling dilute nitric 

 acid, and changed into a yellow floccuient substance. It is inso- 

 luble in concentrated sulphuric acid, and also in boiling alcohol. 

 When treated with caustic potash, a part dissolves with a dark brown 

 colour, and is re-precipitated by acids as a dark brown powder, 

 while the other part remains undissolved as a black powder. 



Mordanted cloth acquires no colour in a boiling solution of xan- 

 thine, if the latter is in its yellow unoxidized state ; but if the solu- 

 tion has become brown by contact with the air, then both the alu- 

 mina and the iron mordant acquire in the boiling solution a brown 

 colour, while the unmordanted parts, which should remain white, 

 become of a brown tint. Hence it follows that xanthine is injurious 

 in madder-dyeing, and must contribute, together with the two 

 resins, in impairing the purity of the colours, and sullying the white- 

 ness of those parts which should attract no colour. To get rid of 

 the xanthine is one object of changing madder into garancine. 



It remains for me to say a few words in regard to the part which 

 the different substances described above play in the process of mad- 

 der-dyeing. I regret to say that in my last report there are con- 

 tained some views on this head, which I have found, on more exact 

 investigation, to be erroneous. The two principal points to be 

 determined are, which is the substance that produces the chief effect 

 in dyeing with madder, and why is a certain proportion of lime, 

 either in the plant or in the dye-bath, necessary for the production 

 of fine and durable colours. In my last report I stated it as my 

 opinion, that both alizarine and rubiacine take part in the process, 

 that rubiacine alone produces no effect, but that when it is in combi- 

 nation with an alkali or an alkaline earth, it forms double com- 

 pounds with the alizarine compounds of alumina and peroxide of 

 iron, and thus increases the intensity of colour in the latter. I have 

 since found that this opinion cannot be sustained, since rubiacine, 

 whether free or combined, produces no beneficial effect in the pro- 

 cess of dyeing, and is therefore no true colouring matter, as the 

 following experiments will show. 



Since the brown precipitate produced by acids in a watery ex- 



