THE CLASSIFICATION OF DYES 175 



seldom use yellowish basic dyes in microtechnique. Thus haema- 

 tein is not used alone. In conjunction with mordants, how^ever, it 

 gives the blue, blue-black, and black colorations that make it pre- 

 eminent among the dyes used in microtechnique. This subject of 

 mordants, however, is so complex that a separate chapter must be 

 devoted to it (p. 207). 



THE ANTHRAQUINONOID DYES 



The simplest dye in this group is alizarine, the chief coloured 

 constituent of madder. This plant-product has been used for dye- 

 ing since ancient times, especially in the production of 'Turkey- 

 red'. The synthesis of alizarine in the laboratory in 1869 gradually 

 put an end to the cultivation of the plant and led to the synthesis 

 of a number of related compounds, some of which are important 

 in the textile industry. The 1:2: 4-trihydroxy-compound is 

 purpunnej which occurs also in madder. Alizarine is too insoluble 



O 



Alizart7je 



for convenient use in microtechnique, but purpurine, though only 

 sparingly soluble in water and alcohol, is used in histochemical 

 tests for calcium. Kernechtrot or 'calcium red' is an anthra- 

 quinonoid dye (or mixture of dyes) of unstated composition, 

 soluble at about 0-25% in water; it can conveniently be used for 

 the same purpose. '^^^ A soluble sodium sulphonate, alizarine red S, 

 can be made from alizarine almost exactly as acid fuchsine is made 

 from basic fuchsine; it is used for staining chromatin, with a 

 mordant, in Benda's ^^' ^^ method for mitochondria. 



Of far more general use in biology than any of these is carminic 

 acid, a dye of quite special interest for several reasons. It is more 

 protean than any other colouring agent, for it can be used as a 

 direct basic dye, is of great value as an acid dye used with a mor- 

 dant, and is also capable of being taken up directly as an acid dye 

 and afterwards changed in the tissues to a basic one. The mor- 

 danted dye is excellent for chromatin, remarkably permanent in 

 Canada balsam, and particularly suited to the colouring of whole 

 mounts. 



