2i8 klO-CHEMICAL JOURNAL 



to vary the colour of paper stained in a mixture of two dyes of 

 opposite character by the addition of electrolytes. This proved not 

 to be so, the reason being the formation of dye-compounds, which 

 appeared to be unaffected by electrolytes. One of these dye-compounds, 

 that of cosin and methylene-blue, is well-known to histologists and is 

 generally regarded as a salt of the methylene-blue base with the eosin 

 acid. Similar compounds are formed by most pairs of acid and basic 

 dyes. They are very insoluble in water though soluble in excess of 

 either component. This latter fact suggests the possibility that they 

 are not true compounds but adsorption-compounds of oppositely- 

 charged colloids. As shown by Biltz mutual precipitation occurs in 

 such cases and solution in excess of either colloid. On this account I 

 have devoted some attention to these bodies. In the first place, it 

 true chemical compounds are formed by a process of double decom- 

 position the inorganic components should be found combined together 

 in the solution. I tested this in the case of anilin-blue and methylene- 

 blue by taking about 3 grams of the latter and the molecular equivalent 

 of the anilin-blue. The precipitate was filtered off and the 

 filtrate evaporated to dryness in order to estimate the chloride 

 present. Unfortunately, by an omission to make a note of the 

 amount of the filtrate from the silver chloride, the exact value was 

 not arrived at. brom the quantity of silver nitrate required to 

 completely precipitate the chloride, it was obvious, notwithstanding, 

 that practically the whole of the chlorine of the methylene-blue was 

 contained in the filtrate. The precipitate was, therefore, a compound 

 of the methylene-blue base with the anilin-blue acid, the sodium of 

 the anilin-blue having at the same time combined with the chlorine of 

 the methylene-blue. I should hesitate to state definitely whether this 

 kind of combination was inconsistent with a process of adsorption, 

 especially in the light of van Bemmelen's results ; on the face of it, at 

 the same time, it appears more like a true chemical compound. 

 Dr. J. H. Scott informs me that he has noticed that a new absorption- 

 spectrum makes its appearance when solutions of eosin and methylene- 

 blue are mixed. I have repeated this observation in the following 

 way : A solution of methylene-blue was taken of such a strength as 



