THE CHEMICAL COMPOSITION OF DYES 165 



for nearly all the dyes used in microtechnique are given in Conn's 

 Biological Stains, ^^'^ which is produced under the auspices of a 

 commission set up in the United States to inspect commercial 

 specimens of dyes used in microtechnique and to certify those that 

 are suitable for use in particular techniques. It includes a 

 number of dyes that are not used in the textile industry. The 

 book gives the wavelength of maximum absorption of many dyes. 



100 



80 - 



52 



I 60 



z 

 < 



40 



20 



400 450 500 550 600 650 



WAVELENGTH, mp 



FIG. 20. Graph showing the transmission of light through a layer i cm 

 thick of acid fuchsine, 0-00293% aqueous.^* 



The use of the full chemical nomenclature for dyes would be 

 intolerably clumsy, because their structure is usually so compli- 

 cated. Arbitrarily-chosen names are used instead. Sometimes, as in 

 the case of light green, the name is a direct statement of the 

 colour of the dye. The colours of the flowers of familiar plants 

 give their names to gentian violet, dahlia, and fuchsine. Eosin, as 

 its name suggests, has a colour reminiscent of the dawn. Sometimes 

 a certain amount of chemical information is conveyed by a name, 

 as for instance by naphthol yellow, which is in fact naphthol with 

 various side-groups. In some cases, however, names that appear 

 to be partly chemical are in fact misleading. Thus methyl blue 

 contains no methyl group and methylene blue no methylene 



