220 BIO-CHEMICAL JOURNAL 



alcoholic solution of a methylene-blue compound, one should see both 

 the methylene blue bands. This is not so, and is due to the fact that 

 in alcoholic solution of methylene-blue the band next the D-line is not 

 to be seen at all distinctly. If the solution showing only the other band 

 is gradually increased in depth or concentration, it is seen that this 

 band broadens out until it reaches the D-line, but there is no indication 

 of a gap between the bands. A fact which confirms the interpretation 

 I have given of the change in the methylene-blue spectrum produced 

 by combination with an acid-dye, is that scharlach R. in weak alcoholic 

 -solution produces the same change, and also a precipitate of a dye- 

 compound. Now scharlach R. is an indifferent dye, not forming salts, 

 but forms an electo-negative colloidal solution ; so that it would seem 

 that here, at all events, the precipitate must be an adsorption compound. 

 A similar state of affairs is to be observed in the case of rosolic acid. 

 Here the possibility of double decomposition is also absent, except, of 

 course, that by mass-action a small proportion of methylene blue 

 rosolate might be formed. The only instance I have found where 

 there is a real change in the spectrum is in the mixture of methylene- 

 blue and picric acid ; picric acid is, however, a powerful reagent, and 

 much weight cannot be laid on this fact as far as concerns the question 

 before us. In mixing methyl-violet and an acid-dye no change is to 

 observed in the absorption spectrum. 



The absence of change in the absorption-spectrum of these dye- 

 compounds does not, of course, exclude the possibility of their being 

 true chemical compounds. 



With regard to the properties of these bodies, they are indiffusible, 

 as such, through parchment-paper. In dilute watery solution they 

 appear to be dissociated ; for, on dialyzing the eosin-methylene-blue 

 compound, for example, it will be seen that after a time eosin begins 

 to pass through, and is ultimately followed by the methylene-blue. 

 Since this observation was made on a solution of the pure body, which 

 had been thoroughly washed with cold water, the eosin and methylene- 

 blue must have passed out as the free acid and base respectively. This 

 dissociation is also shown by the fact that, at 100, the solution is a 

 fairly good conductor of the electrical current, and is electrolyzed by 

 a constant current. 



