CHAPTER XIJ. 



CARMINE AND COCHINEAL STAINS. 



211. Carmine. Carmine is by no means merely carminic 

 acid with at most certain impurities. According to the 

 analysis of LIEBEEMANN (Ber. cL Chem. Ges., Jahrg. J8, 1886, 

 ]>j). 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 advan- 

 tage 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 GKUBLER 

 and HOLLEOEN (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. 



212. 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 car- 

 minic acid, but carminic acid chemically combined with a 

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



