THE CLASSIFICATION OF DYES 177 



Carminic acid is a complex substance. It was shown long ago ^^^ 

 that treatment with hot 50% sulphuric acid splits off a side-group 

 containing six carbon atoms; that subsequent heating to lyo"" C in 

 a mixture of sulphuric acid with one-third its volume of w^ater 

 splits off a carboxyl group ; and that a methyl-trioxyanthraquinone 



CO(CHOH)4CH3 

 OH 



Carminic acid 



is left. The six-carbon side-group is a methyl-pentose sugar, w^hich, 

 when free, has the formula C^HqO^.CH^.^^^ This is an amorphous, 

 bitter-sweet, honey-coloured substance. It is claimed that carminic 

 acid should not be regarded as a glucoside, because the linkage 

 of the sugar with the anthraquinone is through a carbon atom of the 

 sugar, not an oxygen atom.^^^ 



A crude form of carminic acid is often obtained by extracting 

 cochineal with water and precipitating with alum. The product, 

 carmine, contains only about 56% of carminic acid, with a high 

 proportion of protein; also some aluminium and calcium combined 

 with a part of the carminic acid, and other substances in smaller 

 amounts. ^^^ Carmine has the advantage of being considerably 

 cheaper than pure carminic acid. The impurities render it almost 

 insoluble in distilled water. It is suitable for use in ordinary 

 microtechnique, but should not be used when one wants to know 

 exactly what one is doing. 



Alcoholic solutions of cochineal were used in the study of plant 

 stems so early as 1770,^^^ and a few years later Baron von 

 Gleichen ^^'* was feeding ciliates on carmine, in an investigation 

 of their method of nutrition. Carmine was first used as a dye in 

 microtechnique in 1849, by Goeppert and Cohn.^^^ In the 

 following years its use was repeatedly rediscovered, and the 

 general recognition of the value of dyeing in biological micro- 

 technique was due to researches made with this substance. ^^ 



Pure carminic acid was first used in microtechnique in 1884,^** 

 but it was Mayer ^^^ who gave it popularity by introducing some 

 excellent dye-mixtures containing it. 



Carminic acid is soluble in both water and ethanol. It is a fairly 

 strong acid, capable of setting free carbon dioxide from marble. 



M 



