118 Dr. T. Ewan. On the 



with solutions of copper salts. It was found that the solutions of 

 copper bromide, chloride, nitrate, and sulphate possess within the 

 limits of experimental error the same absorption spectrum, while 

 that of copper acetate is different. The difference, however, dimin- 

 ishes with decreasing concentration. 



The absorption spectrum of the dilute solutions of copper salts 

 differs but little from those of concentrated solutions of copper 

 sulphate and nitrate, although in the latter the salts are only partly 

 dissociated into their ions, and in the former much more completely. 

 Observations of a similar nature have led Magnanini* to express the 

 opinion that the absorption spectrum of a substance in solution is to 

 be regarded as independent of the extent to which it is electrolytically 

 dissociated. A more probable explanation is, as Ostwald has pointed 

 out, that the absorption spectrum of the undissociated molecule is 

 very nearly the same as that of the ions, and therefore a change in the 

 degree of dissociation will produce little alteration in the spectrum. 



A case in which the absorption of the undissociated molecule dif- 

 fers considerably from that of its ions was also investigated by 

 Magnanini. j" Solutions of the salts of violuric acid, in which the 

 salt is almost completely dissociated, possess a violet colour ; while 

 Magnanini found the solutions of the free acid to be nearly colour- 

 less, although some 8 per cent, of the acid was dissociated into its 

 ions. J. Wagner,J however, did not succeed in obtaining colourless 

 solutions of the acid. He found that they had the same colour as 

 the solutions of the salts, and a comparison of the intensity of colour 

 in a solution of the free acid and in one of the sodium salt, showed 

 that the intensity of the colour was nearly proportional to the number 

 of ions present. 



In dinitrophenol I have found another case of this sort. The 

 solution containing only undissociated molecules is almost colourless, 

 while solutions containing dissociated molecules are bright yellow. 

 From the measurements of the absorption spectra of these solutions 

 I have been able to calculate the fraction of the dinitrophenol disso- 

 ciated into its ions at different concentrations, and the numbers so 

 obtained are in very satisfactory agreement with those calculated 

 from the electrical conductivity. 



Besides the changes of colour which may be ascribed to the electro- 

 lytic dissociation of the dissolved substance, changes of colour occur in 

 consequence of its hydrolysis by the water. As an example of this kind 

 of change, I have studied the ferric salts, especially ferric chloride. 

 The results are, however, more complicated than those obtained with 



* Magnanini, ' Kendic. Ace. Lincei,' (5), vol. 2, p. 17, 1893; Magnanini and 

 Eentivoglio, ' Kendic. Ace. Lincei,' vol. 7, p. 356, 1891. 

 t ' Zeit-.. Phys. Chem.,' vol. 12, p. 56, 1893. 

 I Wagner. ' Zeit. Phys. Chem.,' vol. 12, p. 314, 1893. 



