Dr. Bolley on the Theory of Dyeing. 501 



nitric acid, as well as strong caustic potash, act strongly on cot- 

 ton by changing its chemical (as in the case of gun-cotton) or 

 mechanical (as in the case of Mercerized cotton) constitution. 



We will, however, endeavour to draw more general conclusions 

 from these experiments when we have considered the views and 

 researches of other writers. 



Verdeil, as we have seen, lays great stress on the different 

 behaviour of wool and silk on the one hand, and cotton on the 

 other, towards mordants. He says, "Wool and silk seem to 

 possess a genuine affinity for dye-stuffs mixed with mordants; 

 while, in order to dye cotton, flax, &c., the dye must necessarily 

 be made insoluble after it has penetrated the substance of the 

 fibres.^' 



It is indeed true that, in dyeing cotton, the mordant is almost 

 universally applied first, and that then, when the base of the 

 mordant has been fixed by the fibre, the dyeing process is pro- 

 ceeded with ; while in the case of silk and wool, and especially 

 the latter, there are dyeing processes in which the mordant and 

 colouring matter are applied simultaneously. It is also unde- 

 niable that comaion alum is often employed by itself in dyeing 

 wool and silk ; whereas, for dyeing cotton, the so-called neutral 

 , or cubic alum, which parts with its base more easily, or the ace- 

 tate of alumina, which is also easily decomposed, or other similar 

 unstable compounds are preferred. 



With reference, however, to the distinction which Verdeil has 

 thus drawn between the processes of dyeing fibres of animal and 

 vegetable origin, it must not be forgotton that, in the so-called 

 "steam-colour-printing," the mordant and colouring matter 

 are applied simultaneously to the cotton fabric; and it will 

 hardly be contended that the fixing of the dye in printing (albu- 

 men printing of coui'se excepted) is different in principle from 

 that in dyeing. 



That common alum with which silk and wool has been im- 

 pregnated is able to attract colouring matter from solutions and 

 pi'ccipitate it on the fibres, depends not on the strength of the 

 chemical affinity of those fibres for the colouring matter, but, as 

 we have seen (see Table), on the fact that they become saturated 

 with the alum, which cotton does not. It is said that in com- 

 bination with animal fibres alum acts more powerfully on solu- 

 tions of colouring matter, so as to form lakes, than it does when 

 by itself or in company with cotton fibres. This assertion, how- 

 ever, is false, though it has run through many handbooks that 

 treat of the properties of dyes. 



It is said, for example, (] ) that solutions of the blue dye of Cam- 

 peachy wood* (the decoction, which alone is of practical import- 

 * Persoz, Traite de I'impression des Tissus, vol. i. p. 630. 



