CARMINE AND COCHINEAL STAINS. 165 



<liition; but for other reasons the addition does not give 

 satisfactory results with the chloride of aluminium solution 

 mentioned above. But it does give good results when 

 combined with an alcoholic chloride of aluminium solution, 

 and thus solves at once the problem of obtaining a red stain 

 and an alcoholic staining fluid. This is described below 

 under the name of Paracarmine. 



If the foregoing explanations of the rationale of carmine 

 staining be compared with the remarks on the theory of 

 staining with haematoxylin given in the next Chapter, an 

 interesting parallelism will be observed. In both processes 

 it is not the colouring matter alone which is active, but the 

 colouring matter combined with alumina. The stain is 

 always got with carminic acid + alumina, or with haematein 

 + alumina ; other substances, such as lime, occasionally 

 playing a part. 



207. The Theory of Staining with Cochineal. According to 

 MAYEK, whose earliest researches are confirmed by his latest 

 Mitth. Zool. Stat. zu Neapel, x, 3, 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 pure aqueous extract contains only traces of 

 lime, the alcoholic none at all. The watery extract made 

 with alum, or cochineal-alum carmine ( 214)* owes its 

 staining power to the formation of carminate of alumina 

 (for which see last ). The tincture made with pure alcohol, 

 on the other hand, contains only the above-mentioned car- 

 minate of some alkali. This carminate alone stains weakly 

 and diffusely (like carminic acid alone). But if in the 

 tissues treated with it it meet with lime salts, alumina or 

 magnesia salts, or even metallic salts capable of combining 

 with it and forming insoluble coloured precipitates in the 

 tis>ues, then a strong and selective stain may result. As a 

 matter of fact, the simple cochineal tincture of Mayer given 

 in 230 does give splendid results with certain objects (i. e. 

 such as contain the salts in question). But it is unfortu- 

 nately equally certain that such objects are rather rare than 

 otherwise, and that with the majority of objects the stain is 

 a very poor one. 



