184 DYEING 



The only important basic mono-azo dye that is important in 

 microtechnique is Janus green B, which, since it contains an azine 

 as well as an azo chromophore, has already been mentioned (p. 181). 



DISAZO DYES 



Bismarck brown has the distinction of being the first synthetic 

 vital dye that was ever used (p. 274). The colour is distinctly 

 unusual in microtechnique. For most purposes it is desirable to 

 use dyes that have either a rather sharply marked absorption-band, 

 so that strong contrasts with other colours can be arranged, or 

 else a general absorption throughout the visible spectrum to give 

 a black that will contrast with an unstained or lightly stained back- 

 ground. Thus browns and yellowish browns are seldom chosen. 

 Bismarck brown Y is a basic dye, convenient for colouring lipid 

 cytoplasmic inclusions during life. 



Trypan blue and its relatives are acid dyes that are used in a 

 very special way in a particular kind of vital work (p. 276). 



Several coloured disazo compounds that are not dyes, because 

 they do not ionize, are very useful in microtechnique for colouring 

 lipids (p. 299). 



TRISAZO DYES 



The only trisazo dye that has found favour in microtechnique is 

 chlorazol black. ^^- It is an acid dye with some basic tendency. 



THE NITRO DYES 



The third and last main chromophore with which we shall be 

 concerned in this book is the nitro-group, -XOg. Colour is due to 

 resonance between two possible positions of the negative electric 

 charge. Nitrobenzene, like other substances containing -NO 2, is 



Nitrobenzene in two resonance positions 



coloured. It is a yellow liquid, but lacks an auxochrome and so is 

 not a dye. Trinitrobenzene possesses three chromophores but is 

 not a dye. If phenol be mixed with concentrated nitric acid, how- 

 ever, a yellow crystalline substance is formed, which, because it 

 contains both -NO 2 (three times) and the auxochrome -OH, is a 



