lyS DYEING 



It forms blueish red or violet salts with metals. The salts with the 

 heavy metals are insoluble, but those with the alkali metals are 

 readily soluble, and indeed carmine itself is soluble in solutions of 

 borax or ammonia. 



The use of carminic acid (or carmine) as a basic dye, and as an 

 acid dye converted in the tissues to a basic one, is described on 

 p. 193. For its use with mordants, see chapter 11 (p. 207). 



THE XANTHENE DYES 



These are to be regarded as derivatives of xanthene, which 

 itself exists in the form of colourless leaflets. A few dyes related to 

 this substance occur in nature, including a yellow one extracted in 



/\/0\/\ 



I I I I 



Xanthene 



India from the urine of cows that have fed on mango leaves ; but 

 the xanthene dyes in the strict sense are synthetic products. A para- 

 quinonoid ring gives colour; an oxygen atom participates in the 



+ 

 /\ O ^\^N(CH3)2 Cl- 



H 



Pyronine G 



linking of two rings. The simplest of those that are useful in micro- 

 technique is the basic dye pyronine G, which, as has already been 

 mentioned (p. 171), is used with methyl green in histochemical 

 tests designed to distinguish DNA from RNA. 



A far more familiar dye in the laboratory, and indeed one of the 

 best known of all that are used in microtechnique, is eosin Y. This 

 is unusual among synthetic acid dyes in not being a sulphonate. 



The auxochromes, as the formula shows, are -Q and -O . It 



is used chiefly as a background dye. Its yellowish red colour and 

 small capacity to overstain fit it rather well for this function, especi- 

 ally when blue dyes such as aluminium-haematein (p. 215) are 

 used for chromatin. A special advantage of eosin is that the dye is 



