496 Dr. Bolley on the Theonj of Dyeing. 



colouring matter to the external surface of the fibre is the prin- 

 cipal cause of their presenting tlie appearance of being coloured. 

 The penetration of the fibre by the dye stuff happens seldom, and 

 the accumulation of the dye in the hollow of the fibre seldomer 

 still. Wool and silk, on the contrary, absorb the dye in all cases, 

 — the adhesion of the lake-dye in the form of an external crust 

 being, however, a very ordinary accompanying phtenomenon, 

 especially in the case of silk. 



We now turn to the other question, whether in the process of 

 dyeing we have to do with chemical attraction ? 



This question has been answered in the affirmative, especially 

 by Chevreul. He, and later still Verdeil, but before them both, 

 Thenard and Roard, have instituted certain experiments with 

 reference to the power of fibres to attract such salts as are used 

 as mordants in dyeing. 



We have first to test the value of these researches, then to 

 extend them, as they are by no means sufficiently numerous, or 

 conducted with necessary care. 



Verdeil imagined he had done enough in burning mordanted 

 silk and wool to an ash and then determining its weight, to 

 draw therefrom the conclusion "that these substances of animal 

 origin possess the property of fixing a certain quantity of the 

 ' base ' of the mordant with which they have been treated.'^ In 

 his researches he left out of consideration — at least the report in 

 the Compfes Renclus mentions nothing of it — (1) the weight of 

 the ash of the wool itself, and (2) whether any of the acid with 

 which the base was combined in the mordant was also taken 

 up. We have given above a summary of the results which he 

 arrived at in the case of silk and wool : cellulose (cotton ?) 

 under similar circumstances does not, according to him, fix the 

 least trace of the base. Besides the omissions above complained 

 of, his method of conducting the investigation does not enable 

 us to form a judgment whether the per-centages so found really 

 represent the amount of mordant taken up ; for who could deter- 

 mine, on the one hand, how much of the mordant adhered to the 

 fibre and resisted the effect of washing, or whether, on the other 

 hand, more had not been taken up and removed by washing ? It 

 may, too, be justly doubted whether cotton mordanted with ace- 

 tate of alumina can have really left no ash containing part of the 

 base of the mordant. The facility with which acetate of alumina 

 is decomposed, either by evaporation, heat, or dilution, is suffici- 

 ently well known, it being precisely this property which makes it 

 fit for a mordant, — that is in this case to yield a basic, almost inso- 

 luble salt. Cotton, linen, or straw, mordanted, or mordanted 

 and dyed in a dyehouse, are found to leave an ash that contains 

 much alumina ; so much so, indeed, that the reduction of a piece 



