THE COAL-TAR COLORS 137 



which has been called fluorescein, while its sodium salt is 

 known as uranin. Solutions of the latter are yellow by trans- 

 mitted light, but bright green by reflected light. This fluo- 

 rescence is so intense that it is distinctly noticeable in ex- 

 tremely dilute solutions; so that this salt has been used to 

 trace subterranean watercourses supposed to connect two 

 neighboring bodies of water, the dye being thrown into one 

 of these and fluorescence being subsequently noticed in the 

 other. The potassium salt of a brominated fluorescei'n is 

 eosin, C2oH 6 O5Br4K2, with a magnificent red and yellow fluo- 

 rescence. These fluorescences disappear on the fibre, but 

 eosin and analogous substances impart very brilliant flesh- 

 tints to silk and wool. 



THE QUINONE DERIVATIVES. These contain the charac- 

 teristic nucleus 



and are almost invariably colored, although they become 

 suitable for dyes only when they also contain several hy- 

 droxyl groups. By far the most important substance of this 

 class is alizarin, which was already mentioned as identical 

 with the active principle of madder. Anthracene, a coal- 

 tar hydrocarbon, is converted into anthraquinone by heat- 

 ing with potassium bichromate and sulphuric acid; the an- 

 thraquinone is acted upon by fuming sulphuric acid, and 



