140 CARMINE AND COCHINEAL 



(like carminic acid alone). But if in the tissues treated with it, it 

 meets 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. And if the necessary salts be added to the tincture itself 

 there results a solution containing the necessary elements for 

 affording a strong and selective stain with all classes of objects 

 Hence Mayer's later formula, § 276. 



250. General Remarks. Carmine stains are widely used, at the 

 present time, for three general purposes. Because of the great per- 

 manence of this dye in balsam, embryologists use it for staining sections 

 of valuable embryos. As a stain for whole objects, carmine is about 

 the most satisfactory dye because it does not overstain very readily 

 and the excess is easily removed. Aceto-carmine is very widely employed 

 by cytologists for studying, in fresh tissue, the chromosomes of both 

 animals and plants. 



Grenacher's alcoholic borax-carmine may be recommended to the 

 beginner as being the easiest of these stains to work with : or para- 

 carmine, for objects which require a strong alcoholic solution. Carma- 

 lum, or one of the alum-carmines, is also an easy and safe reagent. 



Overstains may in all cases be washed out with weak HCl {e.g. 

 01 per cent.). Alum-solution will often suffice, or, according to 

 Henneguy {Journ. de VAnat. et de la Physiol., xxvii, 1891, p. 400), 

 permanganate of potash. The alum-carmines are fairly permanent in 

 glycerin. None of the acid stains, nor any of Grenacher's fluids, should 

 be used with calcareous structures that it is wished to preserve, unless 

 they be taken in a state of extreme dilution. 



A. AQUEOUS CARMINE STAINS 



a. Acid 



251. Alum-carmine (Grenacher, Arch. mik. Anat., xvi, 1879, p. 

 465). An aqueous solution (of 1 to 5 per cent, strength) of common 

 or ammonia alum is boiled for ten to twenty minutes with ^ to 1 

 per cent, of powdered carmine. (It is perhaps the safer plan 

 to take the alum solution highly concentrated in the first instance, 

 and after boiling the carmine in it dilute to the desired strength.) 

 When cold, filter. 



Alum-carmine is an excellent stain. It is particularly to be 

 recommended to the beginner, as it is easy to work with ; it is 

 hardly possible to overstain with it. Its chief defect is that it is 

 not very penetrating, and therefore unsuitable for staining objects 

 of considerable size in bulk. 



This stain must be avoided in the case of calcareous structures that 

 it is wished to preserve. 



TizzoNi {Bull. So. Med. Bologna, 1884, p. 259), Pisenti {Gazz. degli 

 Ospetali, No. 24 ; Zeit. wiss. Mik., ii, 1885, p. 378) and Grieb {Mem. 

 Soc. Ital. Sci., t. vi, No. 9, 1887 ; Zeit. zviss. Mik.,\n, 1, 1890, p. 47), 

 have given modifications of Grenacher's formula which do not appear 

 to us rational. 



