hand, the iron alum is mixed with the dye, and the mixture used for 

 staining sections of tissue. 



Carmin is of considerable historic interest. It was used as early 

 as 1839 by Ehrenberg, altho as we have seen (p. 7) not exactly for 

 histological purposes. It was also employed in 1849 by Goppert 

 and Cohn, by Corti in 1851, and by Hartig in 1854-8, these being 

 the first uses of dyes in histology. It is still a valuable stain today, 

 in spite of the enormous variety of synthetic dyes now available. 

 On account of its freedom from toxicity it is useful for staining by 

 injection. It is much used for staining in bulk, particularly in 

 embryological work. A well known formula is Schneider's aceto- 

 carmin, which is a valuable chromatin stain for fresh material in 

 smear preparations. Alum carmin was used by Grenacher for 

 similar purposes. Carmin is only slightly soluble in water at a 

 neutral reaction; so solutions must be either acid (like the two 

 above) or alkaline. Three alkaline formulae are of considerable 

 use: ammonia carmin, which has been used both for injection and 

 for staining sections; soda carmin, used primarily for injection; and 

 Mayer's magnesia carmin, useful either for sections or for staining 

 in bulk. Alcoholic solutions are also used: Grenacher's borax car- 

 min (or as modified by Mayer) being a splendid nuclear stain for 

 sections; and the hydrochloric carmin of Mayer serving both for 

 sections and for staining in bulk. A special formula containing 

 aluminium chloride (known as muci-carmin) has been proposed by 

 Mayer and is used for staining mucin. In double staining it is 

 sometimes used with indigo carmin; but most often with picric 

 acid. Picro-carmin is a very well known combination used for 

 double staining effects in sections, particularly for nervous tissue; 

 it stains nuclei red and cytoplasm yellow. 



One of the most recent and important uses of carmin is in Best's 

 carmin stain for glycogen. The method is simple and the result 

 beautiful, the red glycogen standing out in sharp contrast to the 

 blue of the nuclei after staining in alum haematoxylin. The stain 

 is permanent; the method is of much importance both to the path- 

 ologist and to the histologist. 



Carminic acid. The dye principle of carmin and cochineal is 

 carminic acid. This product is obtained by extracting the insect 

 bodies with boiling water, treating the extract with lead acetate or 

 barium hydrate, and then decomposing the lead or barium carmin- 

 ate with sulfuric acid. The exact composition of carminic acid is 

 still somewhat uncertain; so far as known, it is: 



CH3 OH 



I CO I CeHiiOs 



HO / CO f OH 

 COOH OH 



93 



