STAINING. 181 



It is important to keep clearly in mind that in speaking 

 of dyes the terms " acid " or " basic " refer to the characters 

 of the colour acids or colour bases, and not to those of the 

 salts. An " acid " dye may have a neutral or alkaline 

 reaction (e. g. picrate of ammonia), and vice versa. 



Basic dyes are generally easily soluble in alcohol, less 

 easily in water ; whilst the contrary is the case for acid 

 dyes. The free colour bases or colour-acids are generally 

 less soluble in water than their salts, for which reason they 

 are so little used that they are hardly to be found in com- 

 merce. It follows that such histological formulse as depend 

 on setting free a colour-acid from its salt {e.g. as by precipi- 

 tating it from eosin by means of alum, as advised by Ranvier 

 and Wissotzky) are irrational. Colour-bases or colour-acids 

 may themselves be colourless. 



The stain given by acid dyes is fast to acids, and may be 

 intensified by them ; whilst basic dyes are washed out by 

 acids, but intensified by alkalies. 



" Neutral " dyes are compounds of a colour-base with a 

 colour-acid. They are seldom or never prepared industrially, 

 the only example that I can find mentioned in Benkdikt and 

 Knecht's Chemistry of the Goal-tar Colours being artificial 

 indigo. They are prepared for histological purposes by 

 mixing the aqueous solutions of a basic ||and an acid dye. 

 For instance, by mixing the acid picrate of ammonia with 

 the basic hydrochloride of rosanilin, you can bring about the 

 formation of sal ammoniac and picrate of rosanilin, which is 

 a " neutral " colouring matter. They are generally insoluble 

 in pure water, and hence precipitate when the mixture is 

 made, but may be got to redissolve by adding an excess 

 of the acid colour, or of the basic, and are always soluble 

 in alcohol. They can also, as is often done, be formed 

 in the tissues themselves by staining first with an acid dye, 

 and then bringing the preparation, without first washing out, 

 into a basic dye. 



Ehelich and Lazabus (" Die Ansemie," Wien, 1898, p. 26) state that 

 the basic dyes xaethyl-green, methylen-blue, amethyst violet (also 

 pyronin and rhodanjin), and the acid dyes Saurefuchsin, Orange G, 

 and Narcein, are peculiarly favourable for making neutral mixtures. 



See further as to the " neutral " colours, Rosin, " TJeber eine neue 

 Gruppe der AnilinfarbstofEen," in Berliner hlin. Wochenschr., xii, 1898, 



