On the Absorption Spectra of Dilute Solutions. 117 



" On the Absorption Spectra of Dilute Solutions." By THOS. 

 EWAN, B.Sc., Ph.D., 1851 Exhibition Scholar in Chemistry 

 in the Owens College. Communicated by Professor H. B. 

 DIXON, F.R.S. Received April 7, Read June 21, 1894. 



The changes which occur in the colour of a solution when its con- 

 centration is changed, are closely connected with the simultaneous 

 changes which occur in the molecular condition of the dissolved sub- 

 stance, and a study of these changes of colour may be expected therefore 

 to furnish valuable information as to the molecular condition of the 

 dissolved body. In a paper published in the ' Philosophical Magazine ' 

 for 1892, I described some experiments on the absorption spectra of 

 solutions of copper salts, which were made from this point of view. It- 

 was found that with increasing dilution the spectra of the different 

 salts examined tended to become the same. The solutions experi- 

 mented with were, however, tolerably concentrated, which makes the 

 interpretation of the results difficult, as, according to Kundt's rule, 

 the absorption spectrum of a substance varies to some extent with 

 the dispersive power of the solvent in which it is dissolved, and, 

 therefore, when working with strong solutions there is always the 

 possibility that a change in the absorption, produced by changing 

 the concentration of the solution, may be due to the alteration in 

 dispersive power. In order to a^oid this uncertainty, I have con- 

 fined my attention almost exclusively to dilute aqueous solutions. 

 In such solutions differing very little from pure water a change 

 in colour can only be ascribed, according to our present knowledge, 

 to some change in the nature of the dissolved substance, and there- 

 fore definite information as to the latter may be obtained. 



A very large number of observations on the spectra of dilute solu- 

 tions of salts of the same metal with different colourless acids and 

 of the same acid with different colourless metals have been made by 

 Ostwald ('Zeit. Phys. Chem.,' 9, 579, 1892). He measured the posi- 

 tion of the absorption bands in the spectra, and also photographed 

 them. Both methods of observation showed that the absorption 

 spectra of all salts containing the same coloured metal or acid were 

 identical. This is a necessary consequence of the electrolytic disso- 

 ciation theory of Arrhenius. For at the concentrations employed, 

 the salts being almost completely dissociated into their ions, the 

 colour of all the salts containing the same coloured metal, for example, 

 will be due to the same ion. 



As, however, no photometric measurements of the absorption 

 spectra of very dilute solutions have been made from this point of 

 view, I thought it not superfluous to make a series of measurements 



