CHAPTER XL* 



STAINING. 



201. Dyes. In practice, the staining materials are usually of the 

 nature of neutral salts. But in the so-called " basic " dyes it is the 

 base or the cation that consists of the complex organic compound 

 possessing colour ; in the " acidic " dyes it is the acid or anion that 

 is the staining agent. In the former case the colour-base is com- 

 bined with a simple acid, generally hydrochloric or sulphuric, but 

 sometimes acetic. In the latter case the colour-acid is combined 

 with an inorganic base, usually sodium. For example, the dye 

 called fuchsin is the hydrochloride of the base rosaniline, and its 

 staining properties are clearly due to the latter. Acid-fuchsin, on 

 the other hand, is the sodium salt of a sulphonic acid derived from 

 fuchsin, and its coloured constituent is present as the acid. The 

 free colour-base or colour-acid is in most cases insoluble in water, 

 although it may exist therein in the colloidal state. These are 

 frequently colourless in themselves. It follows that the addition 

 of acids to formulae for " acidic " dyes or bases to " basic " dyes is 

 devoid of a rational foundation. 



Although the terms " acidic " and " basic " serve to indicate an 

 important difference between dye-salts, their careless use may lead 

 to unwarranted conclusions. Thus, if a cell-constituent takes up a 

 basic dye, it does not follow that this body has the chemical nature of 

 an acid. It may have, but substances other than acids are stained 

 by basic dyes, as we shall see below. 



A " neutral " dye-salt may clearly also be formed by combination 

 between a colour-base and a colour-acid. These compounds are for 

 the most part insoluble in w^ater, although soluble in alcohol. Being 

 of high molecular dimensions, they have the properties of colloids, 

 amongst others, that of forming permanent colloidal solutions in 

 the presence of excess of either component. Moreover, the compo- 

 sition of the dye that is precipitated varies according to the relative 

 proportion of the two reagents in the solution. It is, therefore, not 

 a simple salt, but rather a " colloidal-complex " or " adsorption- 

 compound." These dyes have limited use, more especially in 



* W. M. Baylies. 



