THE CLASSIFICATION OF DYES 185 



dye. It is trinitrophenol or picric acid, the only substance that is 

 used both as a fixative (p. 96) and a dye. It is one of those acid 

 dyes that are actually acids, like carminic acid, for instance. Its 



OH 



OaNj^NNOa OaN^NNOa 



NO2 NO2 



Trinitrobenzene Picric acid 



salts are seldom used in microtechnique. It is a far stronger acid 

 than phenol, and this to some extent militates against its use as a 

 background dye, for which its pale yellow colour otherwise fits it ; 

 for it has a tendency to remove basic dyes. Its acidity, however, is 

 not harmful to the action of other acid dyes, and it is useful in 

 such mixtures as picro-nigrosine (p. 236). 



The only other nitro-dyes that are at all frequently used in 

 microtechnique are naphthol yellow and aurantia. The latter is 

 the background dye in Kull's ^"^ method for mitochondria. 



Orcein is a dye of considerable interest. It has long been used for 

 showing elastin (p. 233) and latterly has come into favour in tech- 

 niques for showing chromosomes in smeared preparations. It 

 cannot be included in the classification of dyes, because its struc- 

 tural formula is unknown. 



Since ancient times dyes have been prepared from lichens. The 

 modern usage, however, dates from the fourteenth century, when 

 a Florentine merchant began to make them on a large scale. 

 Rocella, the genus most commonly used in the preparation of 

 orcein, commemorates the name of his family. The lichens of this 

 genus, commonly called orchil- or archil-weed, grow chiefly on 

 rocks near the sea-shore in the warmer parts of the world. They 

 form tufts of bluish-grey or whitish strap-shaped fronds, up to 

 6 inches long. Several species are usable as raw materials for the 

 preparation of orcein. In Scandinavia and other cool climates 

 another lichen, Lecanora tartarea, replaces Rocella^ and indeed 

 many other species can be used.*^^ Lecanora is often called cud- 

 bear, but the familiar names are loosely used. 



These various lichens contain lecanoric acid, which splits to 

 produce orcinol when the plants are boiled with water. ^^^ It will 

 be noticed that orcinol is resorcinol with a methyl group attached. 

 It crystallizes as colourless, hexagonal prisms, freely soluble in 

 water. The substance can also be prepared synthetically. ^^^ 



