CHAPTER XII. 



CARMINE AND COCHINEAL STAINS. 



213. Carmine. Carmine is by no means merely carminic acid 

 with at most certain impurities. According to the analysis of 



LIEBERMANN (Ber. d. Chem. Ges., Jahrg. 18, 1886, pp. 1969 1975) 



it is a very peculiar alumina-lime-protein compound of carminic acid, 

 a true chemical compound from which at all events aluminium and 

 calcium can no more be absent than sodium from salt. It results 

 from the researches of MAYER (Mitth. Zool. Stat. Neapel, x, 1892, 

 p. 480) that in the processes of histological staining (not of industrial 

 dyeing) the active factors of the compound are, besides the carminic 

 acid, always the alumina, and in some cases the lime. The other 

 bases are inactive ; the nitrogenous matters, so far as they have any 

 influence at all, are an obstacle, as it is they that give rise to the well- 

 known putrefaction of the solutions. 



This being so, it follows that carminic acid may, if desired, be taken 

 as the basis of staining solutions instead of carmine. Staining solutions 

 thus prepared do not give essentially better stains than those made 

 with carmine ; but have the advantage of being of more constant 

 composition. For carmine is a product which varies greatly from 

 sample to sample. 



Carminic acid of sufficient purity is furnished by GRUBLER and 

 HOLLBORN (or C. A. F. KAHLBAUM, in Berlin). It is soluble in water 

 and weak alcohol (that of 70 per cent, only dissolves less than 3 per 

 cent.). It cannot be used alone for staining, as it only gives in this 

 way a weak and diffuse stain. 



214. Cochineal. According to MAYER (Mitth. Zool. Stat. Neapel, 

 x, 1892, p. 496), the active principle of extract or tincture of cochineal 

 (as used in histology) is not free carminic acid, but carminic acid 

 chemically combined with a base which is not lime, but some alkali. 

 The watery extract made with alum, or cochineal-alum carmine 

 ( 216), owes its staining power to the formation of carminate of 

 alumina (last ). The tincture made with pure alcohol, on the other 

 hand, contains only the above-mentioned carminate of some alkali. 

 This carminate alone stains weakly and diffusely (like carminic acid 

 alone). But if in the tissues treated with it it meets with lime salts, 



