Dr. Bolley on the Theory of Dyeing, 483 



All the phsenomena of attraction may be divided into three 

 groups. 



The first comprises those whose results we call chemical com- 

 binations, consisting of the approximation of atoms or atom 

 groups, where the physical properties of the product are entirely 

 difiFerent from those of its components. 



The second consists of those immediate approximations of 

 atoms or atom groups which can only occur when the form and 

 size of the atoms render it possible; it includes what are ordi- 

 narily called the phsenomena of cohesion. Persoz* explains how 

 these two classes of phsenomena, generally described as altogether 

 distinct, are really very similar. 



The third class of pha3nomena, which has been less accurately 

 investigated, stands further removed from either of the former 

 than they from each other. It includes the attraction between 

 gases and porous bodies, the union of gases in the presence of 

 porous bodies (catalysis), the absorption of certain gases in water, 

 the separation of colouring matter from solutions by charcoal, 

 and the decomposing efi^ect of charcoal on solutions of salts. 



The question which naturally follows these introductory ob- 

 servations, viz. to which of these three classes of phsenomena 

 the fixing of colours on tissues and yarns belongs, is answered as 

 follows. 



Two stages must be distinguished in the process of dyeing. 



1st. The colouring matter is to be rendered insoluble. This 

 is eflPected either by the chemical change which the dye itself 

 undergoes (as oxidation in the case of protoxide of iron and 

 indigo-white), or by precipitation (safflower), or by the evapora- 

 tion of the solvent (sulphide of arsenic from ammoniacal solu- 

 tions), — all examples of substantive dyes ; or by the production 

 of chemical combinations of the nature of salts (lakes), of which 

 the colouring matter is itself a constituent — dyeing with mor- 

 dants and the so-called adjective dyes. There is no doubt that 

 these processes belong altogether to the class of chemical attrac- 

 tions. 



2nd. The second stage of the dyeing process is the union of 

 the insoluble colouring matter with the fibre. This phsenomenon 

 Persoz places in the second class of attractions, that is to say, he 

 considers it as due to superficial attraction t. 



* In detail in his work entitled "Introduction a I'etude de la chimie mo- 

 leculaire," Strasbourg, 1839, — a work in which many valuable ideas on mole- 

 cular chemistry (that, for example, concerning the atomic volume of bodies 

 of greater or less fluidity) are thrown out, which have since been partly 

 worked out to their results by others. 



t His words are, " Selou nous, cette adherence des couleurs est due a 

 unc juxtaposition immediate, la uiatiere coloraute se deposant, nou dans 

 les pores des brins de coton, de laine, et de sole, mais a Itur surface, qui, 

 2 12 



