96 CAEMINE AND COCHINEAL STAINS. 



staining be compared with the remarks on the theory of 

 staining with haematoxylin given below ( 174) 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. 



146. The foregoing considerations deal with the theory of 

 staining with Carmine ; we have now to consider 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 ( 154), owes its staining power to the formation 

 of a carminate of alumina, the general properties of which 

 have been discussed above when treating of the theory of 

 carmine staining ( 145). 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 meet with lime salts, alumina or magnesia 

 salts, or even metallic salts capable of combining with it and 

 forming insoluble coloured precipitates in the tissues, then a 

 strong and selective stain may result. As a matter of fact, 

 the simple cochineal tincture of Mayer given in 172 does 

 give splendid' results with certain objects (i. e. such as contain 

 the salts in question). But it is unfortunately equally certain 

 that such objects are rather rare than otherwise, and that with 

 the majority of objects the stain is a very poor one. 



But if the necessary salts be added to the tincture itself, 

 then a solution ought to result containing the necessary 

 elements for affording a strong and selective stain with all 

 classes of objects. This proves to be the case ; whence 

 Mayer's new formula, 173. 



147. General Remarks. What are the carmine stains useful 

 for ? Is it for staining fresh tissues ? With the exception 



