194 CHAPTER XIV. 



obtained from propiolic acid. It appears, however, to be 

 established that neutral colours are frequently formed by the 

 mixture of the solutions of an acid colour and a basic colour. 

 They are generally insoluble in pure water, and hence pre- 

 cipitate when the mixture is made, but may be got to re- 

 dissolve by adding an excess of the acid colour, or of the 

 basic, and are always soluble in alcohol. 



See further as to the " neutral " colours thus obtained, ROSIN, " Ueber 

 eine neue Gruppe der Anilinfarbstoffen," in Berliner Tdin. Wochenschr., 

 xii, 1898, p. 251 ; Zeit. f. wiss.Mik., xvi, 2, 1899, p. 223 ; Journ. Roy. Mic. 

 Soc., 1899, p. 547. 



Now, according to Ehrlich, the basic colours are in general 

 chromatin stains, that is, they have a special affinity for 

 the element of nuclei known as chromatin, so that they are 

 mostly sharp nuclear stains. The acid colours, on the other 

 hand, are, according to him, in general plasma s.tains, that 

 is, they have a special affinity for cytoplasm and intercellular 

 .substances. And lastly the neutral colours exhibit special 

 affinities for certain cell-contents ; amongst them are found 

 some important granule stains. 



I think that that is a generalisation which requires to be 

 supplemented by a good deal of explanation and restriction. 

 In practical histology we have to take account not only of 

 the " affinities " of a dye for this or that cellular element, as 

 they are manifested in progressive staining under narrowly 

 limited conditions ; we have also to take account of the 

 resistance of tlie stain to the liquids employed for washing, 

 for dehydration, for clearing ; in short, we have to take into 

 account the way in which the dye behaves when employed 

 as a regressive stain. This is of peculiar importance in the 

 case of the coal-tar colours, seeing that they are largely 

 used for the regressive staining of sections destined to be 

 dehydrated by alcohol and mounted in balsam. Now Erhlich's 

 experiments take no account of these conditions. He worked 

 with 'cover-glass preparations" of isolated cells, such as 

 blood iind lymph cells, and was thus able to avoid the pro- 

 longed washing necessary for most sections, and to suppress 

 ;i It ogether the dehydration by alcohol, his cover-glass prepara- 

 tions being simply dried after staining in a stove. In con- 

 sequence, his chemical categories of basic colours and acid 



