THE CHEMICAL COMPOSITION OF DYES 



o o 



95 



O 



Anthraqiiinone 



OH 



Alizarine 



CO(CHOH)4CH.<. 

 OH 



Carminic acid 



cularly valuable dye in biological work. Carminic acid is soluble 

 (unlike alizarine) in distilled water, and also in ethanol. It occurs 

 naturally in the fat-body of the wingless females of the coccid 

 Dactylopiiis cacti. This is a scale-insect that sucks the juices of the 

 succulent plant Nopalea coccinellifera. The latter is cultivated in 

 various subtropical parts of the world to provide the insect with 

 food. The dried females constitute cochineal. Carmine is a crude 

 form of the dye, produced by precipitating an aqueous extract of 

 cochineal with alum (potassium aluminium sulphate). It is com- 

 posed mainly of carminic acid bound to aluminium and to pro- 

 tein derived from the insect. Carmine is insoluble in distilled 

 water; but solutions can be obtained (e.g. by the addition of acid 

 or alkali), and the crude dye is usable in microtechnique. It is 

 best to use the pure acid whenever one wants to know exactly 

 what reactions are occurring. 



One of the main uses of carmine in microtechnique is explained 

 in the chapter on the use of mordants (p. 122). 



Xanthene dyes. In these dyes, carbon and oxygen form links 

 between two rings. Not many members of this group are com- 

 monly used in microtechnique. Eosin, however, is particularly 

 familiar. It is a valuable 'background' dye: that is to say, it is 

 anionic, pale, diffuse in action, and used to give contrast with 

 objects picked out vividly in another colour by another dye. 



