On the absorption spectra of dilute solutions

The changes which occur in the colour of a solution when its concentration is changed, are closely connected with the simultaneous changes which occur in the molecular condition of the dissolved substance, 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 experimented 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.