484 Dr. Bolley on the Theory of Dyeing. 



It Is obvious that thuSj together with the second of the ques- 

 tions mentioned at the commencement of this essay, on which 

 the whole subject turns^ the first also is most unequivocally 

 answered. 



The practical illustrations which Persoz adduced in support 

 of his own views, he also employed against a theory of Walter 

 Crum's on the subject of cotton-dyeing. We shall therefore give 

 a short sketch of Crum's theory before we notice the arguments 

 of Persoz. 



According to W. Crum*, the cell of the cotton fibre, in its 

 fully developed state, is flattened throughout its entire length, 

 except at the edges, which remain raised and tubular. He 

 appeals to the microscopic observations of Thomson of Clitheroe, 

 and Bauer (which, however, all the more modern researches show 

 to be incorrect), and contends that the mordant penetrates into 

 these canals by means of the pores in their sides, and there de- 

 composes and dries up; and that the solution of colouring mat- 

 ter afterwards applied follows the same course, forming a lake 

 with the mordant base. To the repeated washings which are 

 found so necessary in cotton-dyeing, he atti-ibutes the effect of 

 removing that part of the mordant which has adhered to the ex- 

 ternal surface of the fibre. In conclusion, he draws a comparison 

 with the attractive power of charcoal for colouring matter and 

 salt solutions, and explains several dyeing operations (those, 

 namely, in which mordants may be dispensed with) as attribu- 

 table to a power in the fibre analogous to that of charcoal. 



Persoz very justly asks, how is the printing of cottons, when the 

 pigment is thickened with some viscous substance, to be explained 

 on W. Crum's theory ? It is impossible to believe that the semi- 

 fluid mass can penetrate the pores of the fibres and expel the air 

 from the interior tubular space. Stuffs to be dyed indigo-blue 

 are often first prepared with a thick mixture of stiff paste and 

 blue vitriol, in which case it is impossible for the dye to penetrate 

 the pores in the sides of the cells. The process of the so-called 

 enlevage, or decolouring printing, would be hard to understand 

 if the colouring took place in the interior of the fibre instead of 

 on the outside surface. In the case both of indigo-blues and 

 nankeens, the observation may easily be made, that, by repeated 

 washings and dryings, the indigo or oxide of iron may be conti- 

 nually diminished. A piece of blue cotton cloth was laid for a 

 week in clear running water, and it was at last found that the 

 colour was almost washed out. How is it that stuff dyed a full 



ainsi qu'on peut s'eii assurer au microscope, se compose de facettes assez 

 semblables pour la laine aux ecailles de poisson." 



* " On the Maimer in which Cotton unites with Colouring Matter," 

 Philosophical Magazine for April, 1844. 



