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Proceedings of the Royal Society of Edinburgh. [Sess. 
contains many free ions of Cu and Cl, but also a certain amount cf un- 
dissociated molecules of CuCl 2 , will show the colour which results from 
mixing the blue of the copper ions with the yellow of copper chloride. 
Agreeably with this conclusion, it is found that very dilute solutions of 
copper chloride — which contain but little undissociated salt — are blue, like 
solutions of other copper salts ; but the more concentrated the solutions 
become, and therefore the greater becomes the number of the non-dissociated 
molecules, the more does the colour incline towards green. Addition of 
hydrochloric acid, or raising the temperature, acts in the same way as 
increasing the concentration ; in both cases, the number of copper ions 
decreases, and the number of the undecomposed molecules of copper chloride 
increases.” 
Whetham also ( Theory of Solution, 1902, p. 334), referring to the 
additive properties of electrolytic solutions, says : — 
“ Similar phenomena appear when we study the colour of a salt 
solution, which is found to be produced by the superposition of the colours 
of the ions and the colour of the undissociated salt. If the absorption 
spectra of a series of coloured salt solutions containing a common ion are 
examined, the additive character of the colour is well seen, the absorption 
bands due to the common constituent being unaffected by the presence of 
the other part of the salt. The light transmitted through a solution is 
composed of all these rays which have been absorbed by neither constituent. 
Anhydrous cobalt chloride is blue, while in cold aqueous solution all cobalt 
salts are red. Red, then, is the colour of the cobalt ion, and only appears 
when the salt is more or less dissociated. When cobalt chloride is dissolved 
in alcohol, the conductivity is very low, showing very incomplete ionisation. 
The colour is, accordingly, the blue of the undissociated salt. If we slowly 
add water to this solution, the ionisation gradually increases, and the 
colour changes to purple and then red. An aqueous solution, boiled with 
potassium cyanide, is decolorised, for a cobalticyanide, K 3 Co(CN) 6 , has 
been formed ; the ions of this compound are 3K and Co(CN) 6 ; the free 
cobalt ions no longer exist, and the solution ceases to respond to the usual 
tests for cobalt. That the red colour is really due to the ionisation, and 
not to a hydrate formed between the cobalt salt and the solvent, is 
indicated by the additive nature of the phenomena ; for, like many other 
properties, the colour of non-electrolytes depends on the constitution and 
is not additive.” 
The results given in this series — which are really the first systematic 
measurements on the subject — had shown that ionisation had really nothing 
to do with these changes, but the question of ionisation was only treated 
