CHAPTER XI J, 



CAIISIINE AND COCHINEAL S'J'AINS. 



211. Carmine. — Carmine is by no means merely carmiiiic 

 acid with at most certain impurities. According to the 

 analysis of Liebeemann {Ber. lL Chem. Ges., Jahrg. ]8, 1886, 

 pp. 1969 — 1975) it is a very peculiar aluivina-lime-].jrotein 

 covipound if 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 Mayee {Milth. Zool. Stat. Neapel, x, 1892, p. 480) that in 

 the processes of histological staining {not of industrial dyeing) 

 the aciive 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, 

 he 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 advan- 

 ta.ge of being of more constant composition. For carmine is 

 a product which vai'ies greatly from sample to sample. 



Carminic acid of sufficient purity is furnished by Gktjbler 

 and HoLLBOKN (or C. A. F. Kahlbaum, in Berlin). It is 

 soluble in water and ueah 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. 



212. Cochineal. — According to Mayee {Mitth. Zool. Stat. 

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

 tincture of cochineal (as used in histology) is not free car- 

 minic acid, but carminic acid chemically combined with a 

 base which is not lime, but some alkali. The watery extract 



