l66 DYEING 



group; and azo-carmine is not an azo dye and is not related to 

 carmine. Some dyes have names of fantastic, almost surrealist 

 origin. It was discovered that a particular dye was especially con- 

 venient for the colouring of cotton. In the same year the European 

 powers recognized the existence of the Congo Free State. These 

 unconnected events led to the dye being called Congo red.^^^ 



Despite all that can be said about the lack of consistency in the 

 naming of dyes, the fact remains that the words used are much 

 more easily remembered than the numbers and strings of initial 

 letters that are so often used nowadays in industry for purposes of 

 this kind, without any thought for those who have to try to hold 

 in mind which symbol refers to which object. 



The names of many dyes are followed by letters or numbers. 

 These generally serve to distinguish closely-related dyes. Thus 

 'B' usually means that the dye is more blueish than a related 

 dye, and 'Y' or 'G' (gelb) that it is yellower. 'WS' conveys that it is 

 water-soluble, while a related dye is not. The letters A, B, C are 

 sometimes used as arbitrarily- chosen marks of distinction (for 

 instance, with the azures, p. 268). 



Although dyeing has been and is of such immense service to 

 biology, the number of dyes that are really useful in microtech- 

 nique is not very great. There has been a tendency to try new dyes 

 from mere whim and desire for novelty, and the introduction of 

 many superfluous ones has been recorded from recipe-book to 

 recipe-book as though it were the fruit of wisdom. Dabbling with 

 dyes by persons ignorant of the chemistry of what they are doing 

 has no counterpart in the rest of science and indeed cannot be 

 regarded as a scientific activity. The stricture on this subject 

 reproduced on p. 187 is as applicable today as when it was de- 

 livered by Gustav Mann more than half a century ago. More so, 

 perhaps; for it would be a sobering experience to many present- 

 day dabblers to note the real erudition brought to the subject by 

 such men as Ehrlich and Paul Mayer, right back in the nineteenth 

 century. The person who can show a particular dye to be super- 

 fluous in microtechnique usually deserves better of his colleagues 

 than he who introduces a new one. 



Dyes are classified by their chromophores, but the auxochromes 

 and modifiers occur over and over again in dyes of diiferent groups. 

 It is proper to consider these first, for what can be said about them 

 is of more general application than what can be said about 

 chromophores. 



