THE CHEMICAL COMPOSITION OF DYES 163 



this case -NH2) replacing hydrogen in one or more places.) Dyes 

 that are related to a leucobase of this kind are called triarylmethane 

 dyes. Some of them are of great importance in microtechnique. 



400 



450 



500 550 



WAVELENGTH, m|x 



600 



650 



FIG. 19. Graph showing the optical density of a layer i cm thick of basic 



fuchsine, 0-00062% aqueous.^* 



Another colourless derivative of pararosaniline, in which -OH 

 replaces the hydrogen attached to the central carbon atom, may be 

 regarded as a triaryl derivative of methanol. Compounds of this 



NH, NH, 



H 



NH2 



Leuco-pararosanilinc 



kind are called leucohases. This v^ord has unfortunately been 

 wrongly applied in recent years to Schiff's reagent, which is a 

 colourless derivative of basic fuchsine but not a leucobase (p. 308). 



