CHAPTER XIV. 



ON STAINING WITH COAL-TAR COLOURS. 



262. Basic, Acid, and Neutral Coal-tar Colours. Histologists 

 generally conceive of the coal-tar colours as divided into 

 three groups, according to a principle of classification founded 

 on chemical considerations, and introduced into histological 

 literature by EHRLICH (Zeit. klin. Med., I, 1880, p. 555; 

 Verli. d. BerJ. Phyn. Ges., May 16th, 1879; in REICHERT AND 

 Du-Bois RKYHOND'S Arch. f. Anaf. n. Pliy*., Pliy*. AbtJi., 

 1879, p. 571). These three groups are those of the basic 

 colours, the acid colours, and the neutral colours. By a 

 "basic " colour is meant a compound in which the colouring 

 principle or molecular group to which the compound owes 

 its colouring properties exists as or chemically plays the 

 part of a base combined with a colourless acid. For instance, 

 f uchsin or magenta is a basic colour. It is the hydrochloride 

 of rosanilin, and its colouring properties are due to the 

 rosanilin which exists as a base in the compound, and not to 

 the hydrochloric acid of the compound. By an " acid ' 

 colour is meant a compound in which the colouring principle 

 exists as or plays the part of an acid. The dye known as 

 acid f uchsin or acid magenta (Sauref uchsin) is an " acid ' 

 colour. It is the soda salt of di- or tri-sulphoconjugated 

 rosanilin, that is of rosanilin di- or tri-sulphonic acid, and its 

 colouring properties are due to the rosanilin which exists as 

 an acid in the compound, and not to the soda. Or to take a 

 simpler case, picrate of ammonia is an " acid ' colour in 

 Ehrlich's sense, and its colouring properties are evidently 

 due to the picric acid in it, and not to the ammonia. The 

 neutral colouring matters form a very small group ; the only 

 example that I can find mentioned in BENEDIKT and KNECHT'S 

 Chemistry of the Goal-tar Colours being artificial indigo, 



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