1904.] on Development of the Theory of Electrolytic Dissociation. 561 



A property that is of physical character, but is much used by the 

 analytical chemist, is the colour of the solutions. It has been sub- 

 jected to a rigorous research by Ostwald. At first we will trace how 

 a compound, e.g. fluoresceine, H10C.0O5, behaves if one replaces its 

 hydrogen atoms by other atoms, e.g. metals, iodine, bromine or atomic 

 groups _ (NOo). The curves in the next figure (Fig. 7) indicate 

 absorption-bands in the spectra of the corresponding compounds. A 

 replacement of K^ for Ho in the fluoresceine itself alters the absorption- 

 spectrum in a most sensible manner. This depends upon the pro- 

 perty that the fluoresceine is dissociated to a slight extent, which is 

 in strong contrast to the permanganic acid which will be discussed 

 immediately. Instead of a single absorption-band in the blue in the 

 first case, we find two absorption-bands in the blue-green and the 

 green part of the spectrum for the second. A similar observation 



BCdD Eb F 



Fluoresceine h-. 

 Potassium salt Hi H,q 

 DichlorfNa Cl^ H^ 

 Tetrabromf K-,Br^H(, 

 Tefrajodf Tt He 



salt H2 r, Wg 

 OaethylfetrajodflC^Hi 1^ T, h^ 

 TetrabromdichlorfH: 5r» Cl^ H^ 

 Tetrajoddichlorf H^ T^ CIz H, 

 TttrsbromduniirokJNOih Sr, /^«. 



JJeriycUes of FLuorescurutSiz Czc O5 

 Fig. 7. 



may be made for the tetraiodine-fluoresceine and its potassium salt. 

 In general the figure shows that the spectrum is changed in a very 

 conspicuous manner at the smallest chemical change of the molecule. 

 It might therefore be expected aftej- the old manner of view, that 

 the replacement of hydrogen by a metal in permanganic acid, or 

 of one acid rest by another in the salts of para-rosaniline would 

 wholly change the character of the spectra. This is not the case, 

 as Ostwald has shown. The spectra are wholly unchanged, as 

 Figs. 8 and 9 show. The spectra are all produced by the same 

 substance, viz., the permanganate-ion in the one, the para-rosaniline- 

 ion in the other case. Only in the case of the para-rosaniline salts 

 we observe that the absorption is sensibly weaker in some cases than 

 in others. The weakening depends upon the hydrolysis of the salts 

 of the weak acids, e.g. acetic and benzoic acids. This research of 



