502 Dr. Bolley on the Theory of Dyeing. 



ance^ and not the solution of the pure colouring principle) 

 behave to an alum solution as to an acid, that is to say, they are 

 turned yellow without any precipitate being formed; (2) that a 

 decoction of Brazil wood is turned crimson-i'ed on being treated 

 with alum, but remains clear*; (3) that a decoction of cochineal 

 is turned crimson-red by alum without any precipitate being 

 formed t- Precipitates are, it is assumed, only produced when 

 other causes effect the separation of the alumina, which, forming 

 a flocculent precipitate, carries the colouring matter with it. 



This observation is exceedingly defective. These three solu- 

 tions, like many others, always form precipitates with alum, 

 especially on being heated, if only the precaution be taken of 

 adding a sufficiently small quantity of the latter. If a large 

 quantity be poured in without proper precaution, it is true that 

 onXy discoloration s, similar to those above described,are produced. 

 The solution of chloride of tin, which is often employed in the 

 so-called "physic" of the silk-dyer in conjunction with baths 

 of colouring matter, behaves similarly to alum. 



Finally, Maschke concludes that there is some sort of chemical 

 affinity between animal fibres and colouring matter from the fact 

 that such fibres are coloured by penetration. Against this con- 

 clusion, however, the following objections may be urged. 



Dyers are aware that indigo dissolved in sulphuric acid (liquid 

 blue) coloui's wool as well as silk without a mordant ; but though 

 this admits of no doubt, it nevertheless seems to have escaped 

 observation, that if silk so dyed be steeped in water so long that 

 the latter begins to pass off without being discoloured, and be 

 then plunged for a further period of time in a large quantity of 

 distilled water, it invariably, and, if the water be many times 

 changed, completely loses all traces of colour. The same also is 

 true of wool, though in that case the extraction of the colour is 

 more difficult. 



A solution of cochhical and a decoction of Brazil wood dye 

 silk and wool without a mordant ; but here again the colour may 

 be completely extracted from the latter by long-continued treat- 

 ment with distilled water. 



It must, moreover, be remembered that the conclusion which 

 Maschke and others draw from microscopic observation is pro- 

 bably unfounded. If in dyed silk and wool fibres there is no- 

 thing to indicate that the colouring matter is located in any 

 particular parts, such fibres appearinguniformly coloured through- 

 out, the same is also the case with coloured sheets of glue. The 

 clear dissolved glue, when mixed with coloured solutions of 

 whatever strength, produces a clear gelatine, and on drying 



* Vitalis, Cours El^entaire de Teinture. 



t E. L. Schubarth, Elemente der Technischen Chemie. 



