STAINING 129 



A " neutral " dye-salt may elearly also be formed by com- 

 bination between a colonr-base and colour-acid. These com- 

 pounds are for the most part insoluble in water, 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 composition of the dye that is precipitated varies 

 according to the relative proportion of the two reagents in the 

 solution. 



These dyes are used especially in distinguishing between the 

 various kinds of leucocytes in blood (see Ehrlich and Lazarus, 

 Die Ancemie, Wien, 1898) ; but are coming of later years to be 

 applied to various other purposes. 



238. Influence of Side-chains. Almost any dye can be modified 

 in its characteristics by the introduction of side-chains. Most 

 commonly these are methyl or ethyl radicals, but they may be 

 even more complex [e.g. additional benzene rings). The usual 

 effect of an added side-chain is to deepen the shade or even to 

 change the colour to one of deeper hue. The influence on colour 

 differs according to where these side-chains are introduced in the 

 dye molecule. Thus pararosanilin chloride is changed to rosanilin 

 chloride 



Rosanilin 



by introducing a methyl group directly into one of the benzene 

 rings. Rosanilin is slightly deeper in shade than pararosanilin. 

 The shade may be further deepened by introduction of another 

 methyl group into each of the other two rings, the resulting dye 

 (new fuchsin) being deeper yet, but still red in hue. If, however, 

 the methyl substitution is effected by replacing the H-atoms of 

 the NHa-groups, the hue is changed to violet, a reddish-violet 

 if only two or three methyl radicals are introduced, but a blue- 

 violet if all six amino H-atoms are thus replaced. Finally, it is 



VADE-MECUM. 



