STAINING. 135 



which it figures as an ingredient. Iodine also plays in some 

 staining processes a part which seems only explicable on 

 the supposition that it acts as a mordant. And in some 

 processes an acid dye is made to act as a mordant for a 

 subsequently employed basic dye. 



In some staining processes, however, mordants are inten- 

 tionally resorted to in order to fix the stain. Mordanting 

 has long been intentionally employed in some haematein 

 staining processes, such as the iron-alum process of BBNDA 

 and M. HEIDENHAIN. More lately it has been resorted to for 

 staining with tar colours. 



By combining with the elements of tissues, mordants confer on them 

 an artificial chromatophily. This may take place to so great an extent 

 that the original affinities of the tissues for dyes are not only masked 

 but may be seemingly altered into their opposites, thus producing an 

 " inversion " of their chromatophily. Thus by means of appropriate 

 acid mordants certain basic anilins, which by the usual methods of 

 regressive staining are pure chromatin stains, may be made to afford a 

 pure plasma stain one not affecting chromatin at all, thus giving an 

 " inversion " of the usual stain. 



It would seem that the nuclear stains obtained by carmine and haema- 

 toxylin should in strictness be classed as inversion stains. The colouring 

 principle of carmine, carminic acid, is an acid body, and consequently a 

 plasma stain. But in the form of carmine it is combined with the basic 

 oxide alumina, which by mordanting the acid body nucleiii confers 011 it 

 a basic character and renders it acidophilous, thus producing an inver- 

 sion of the natural staining affinity. So also with the acid dye hsematein. 

 So that we see that whilst basic dyes substantively employed are nuclear 

 stains and acid dyes substantively employed are plasma stains, yet 

 basic colours with acid mordants may give a plasma stain and acid 

 colours with basic mordants may give a nuclear stain. 



206. Metachromasy. Adjective stains are of the colour of the lake 

 formed by the mordant and the colouring principle of the dye. Sub- 

 stantive stains are for the most part of the colour of the solution of the 

 dye employed. But this is not always the case. There is a very small 

 group of dyes, mostly basic tar-colours, which have the property of 

 staining different elements of tissues in different- colours, one of these 

 being the same as that of the solution of the dye, and the other, or 

 metachromatic colour, being the same as that of the free colour-base 

 of the dye. For instance, the red dye, safranin, colours nuclei red, but 

 mucin and the ground- substance of cartilage orange. The blue dye, 

 methyl violet, stains normally blue ; but amyloid matter, Mastzellen, 

 mucin, and cartilage, are stained red by it. Similarly with thionin, 

 dalilia, gentian violet, toluidin blue, etc. 



In these cases the effect appears to be due to chemical reaction. But 



