CHAPTER XIV 



CARMINE AND COCHINEAL STAINS 



248. Carmine. This dye is obtained from the ground-up bodies 

 of the cochineal insect and since the latter varies greatly in the 

 quality of the product yielded, and different methods are used to 

 extract it, the powdered carmine obtained in commerce is extremely 

 variable. For ordinary histological staining this variability may 

 be of little importance, but for techniques requiring a high degree 

 of specificity, such as aceto-carmine, the use of a first-class product 

 is essential for satisfactory results. 



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 venj 

 peculiar alumina-protein compound of carminic acid, a true chemical 

 compound from which at all events aluminiujn 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, 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 advantage of being of 

 more constant composition. 



Carminic acid 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. 



249. 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 from carminic acid 

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

 alkali. The watery extract made with alum, or cochineal alum- 

 carmine (§ 275) owes its staining power to the formation of car- 

 minate 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 



139 



