220 MICROSCOPICAL RESEARCH IN THE 



of alcohol, and this explains why superfluous colouring matter can 

 only be removed from objects by the aid of alcohol of precisely 

 the same degree as that of the tincture. 



Over-staining, which seldom occurs, may be easily corrected 

 by the aid of acid alcohol (i-ioth per cent, hydrochloric acid, or i 

 per cent, acetic acid). Acid makes the tincture lighter, more yel- 

 lowish-red, while the addition of ammonia and other caustic 

 alkalies changes it to deep purple. Still more important is the 

 fact that salts soluble in alcohol give a blue-grey, green-grey, or 

 blue-black precipitate. For example, if a piece of cloth that has 

 been dyed in cochineal and washed, be treated with an alcoholic 

 solution of a ferric or a calcic salt, it will assume a more or less 

 deep blue colour. 



As the salts present in the living organism are seldom, if ever, 

 fully removed by preservative fluids, but in some cases even in- 

 creased, it win often happen that an object, though stained in the 

 red fluid, comes out blue, precisely as when stained with haema- 

 toxylin. Such a result cannot, however, be obtained in the pre- 

 sence of acids, nor in the absence of inorganic salts ; under these 

 conditions the colour is always red. It is not possible, therefore, 

 to know what colour an object will ultimately present. 



Very often the different tissues of one and the same object 

 present unlike colours. In the embryos of Lumbricus^ Kleinen- 

 berg found the walls of the blood-vessels red, their contents dark 

 blue. Glandular tissues, or their contents, are frequently stained 

 grey-green. 



Objects treated with chromic or picric solutions, or with alco- 

 hol, usually stain without difficulty ; but osmic acid preparations 

 should be bleached before staining. Cochineal does not colour so 

 intensely as haematoxylin, and hence the latter often gives more 

 satisfactory results in the case of large objects stained /;/ ioto. 



As before pointed out, alcohol causes the salts contained in sea 

 water to be precipitated, thus forming a crust on the exterior of 

 the animal which interferes with the staining process. It is there- 

 fore necessary to treat marine animals that have been preserved 

 in strong alcohol with acid alcohol (i-io parts hydrochloric acid 

 to I, GOO parts 70 per cent, alcohol), and then carefully wash in 

 pure 70 per cent, alcohol before staining with cochineal. 



