62 ANILIN COLOURS GIVING INDIRECT NUCLEAR STAINS. 



The length of time necessary for washing out to the precise 

 degree required varies considerably with the nature of the 

 tissues and the details of the process employed ; all that can 

 be said is that it generally lies between thirty seconds and 

 two minutes. 



In more than one of the methods presently to be described treatment 

 with chromic acid or with iodine forms part of the washing out process. 

 The rationale of this is somewhat obscure ; the most probable point of view 

 appears to be that the chromic acid acts as a mordant on the chromatin and 

 helps it to retain the stain. It is known on the one hand that chromic acid 

 precipitates safranin from its solutions, so that by admitting a special 

 affinity of chromic acid on the other hand for chromatin, and especially for 

 chromatin in the kinetic state, the explanation is hypothetically complete. 



The iodine in Bizzozero's (Gram's) process also appears to act as a fixative 

 of the colour. 



It has been ascertained by HENNEGUY that permanganate of 

 potash acts as a mordant for coal-tar colours. For the 

 manner of employing it see 106. 



98. Substitution. It was stated above that washing out is 

 generally done with alcohol. There exists another mode of 

 washing out that is both of practical importance and of great 

 theoretical interest, one anilin stain may be made to wash 

 out another. Thus methylen blue and gentian violet are dis- 

 charged from tissues by aqueous solution of vesuvin or of 

 eosin; fuchsin is discharged from tissues by aqueous solution 

 of methylen blue. The second stain " substitutes " itself for 

 the first in the general "ground" of the tissues, leaving, if 

 the operation have been successfully carried out, the nuclei 

 stained with the first stain, the second forming a "contrast" 

 stain. It appears from the interesting paper of EESEGOTTI 

 in Zeit. f. wiss Mik., v, 3, 1888, p. 320, that it may be stated 

 as a very general rule that colours that do not give a nuclear 

 stain by the indirect method will wash out those that do. 

 Thus he found that 



Safranin, Dahlia, Methyl Violet, Gentian Violet, Rubin, 

 Victoria Blue, Magenta, Basic Fuchsin, are washed out by 

 the following : 



Congo, Methyl Green, Iodine Green, Nigrosin, Methylen 

 Blue, Orange, Ponceau, Acid Fuchsin, Aurantia, Cyanin, 

 Eosin, Methylic Eosin, Magdala Red, Bordeaux, Vesuvin. 



