CHAPTER XL 121 



distinguishing between various kinds of leucocytes in blood (see 

 EHRLICH and LAZARUS, Die Anaemie, Wien, 1898). 



Since the acidic dyes are salts of fairly strong sulphonic acids 

 with strong bases, they are electrolytically dissociated in solution to 

 a large extent. Thus their solutions contain coloured anions, 

 colourless cations (usually Na), together with undissociated salt. 

 They are only hydrolytically dissociated to a negligible degree, if 

 at all. The basic dyes, on the other hand, are salts of weak bases 

 (amino- or imino-derivatives) with strong acids. Although electro- 

 lytically dissociated as salts, so that their solutions contain coloured 

 cations and undissociated salt together with colourless anions, they 

 also undergo hydrolytic dissociation to a notable degree. Thus 

 they contain free colour base and free colourless acid in addition 

 to their ions. 



It is important to remember that commercial samples of dyes 

 contain mineral salts, as a rule, sodium chloride or sulphate, some- 

 times as much as 30 per cent, or more. This must be borne in mind 

 in statements as to their properties. For instance, it is often said 

 that the Congo-red dyes are direct dyes for cotton fibre. This is 

 only the case in the presence of salts, as will be shown presently. 



Details of the chemical composition of different dyes are beyond 

 the scope of this book. The reader is referred to CAIN and THORPE'S 

 Synthetic Dyestuffs, 1913. 



202. The Nature of the Staining 1 Process. From what has been 

 said in the preceding paragraph it will be realised that a solution of 

 a dye is a complex system from a physico-chemical standpoint. 

 Moreover, the structures to be stained are present as separate 

 phases, solid or liquid, of a heterogeneous system. It is clear, 

 therefore, that the properties of boundary surfaces must be taken 

 into consideration, in addition to differences of chemical composition 

 and of colloidal state. Much discussion has taken place with respect 

 to the process of dyeing, and various theories of its nature as being 

 essentially chemical or essentially physical, in the sense of adsorption, 

 mechanical or electrical, or in the sense of solid solution, involving 

 partition between the solution and the tissue elements according to 

 relative solubility of the dye therein, have been advocated. It is 

 probable that all these factors play their part in varying proportion 

 and that no one theory alone can explain all the facts. 



We shall be in a better position to appreciate the complexity of 

 the conditions present if we examine, to begin with, the case of a pure 

 substance, cellulose, in relation to pure solutions of an acidic and a 

 basic dye respectively. 



