SALTS OF COPPER. 57 



In general, the absorption spectrum of copper salts in the region of 

 the spectrum investigated is much simpler than that of cobalt salts, inas- 

 much as it presents only two or at most three absorption bands. Of these, 

 only the one at X 4730 in acetone solutions lies wholly in the spectral region 

 studied; the band in the ultra-violet is what might be termed one-sided, 

 no region of transparency on its more refrangible side having ever been 

 found. The band in the red is, however, strictly a band, a region of trans- 

 parency existing in the infra-red. The behavior of this band throughout 

 strongly suggests the green band of cobalt salts in solution, while the ultra- 

 violet absorption is somewhat different from anything we have found 

 thus far, resembling more nearly the absorption of iron salts, to be dis- 

 cussed in the next chapter. 



Since the absorption in the ultra-violet decreases rapidly with dilution, 

 when the product of concentration and depth of layer is kept constant, 

 it seems reasonable to suppose that the copper ion has little or nothing to 

 do with it, and hence that it must be ascribed to the molecules; but as the 

 absorption decreases with dilution, even when molecules are kept constant, 

 without, however, entirely disappearing (as was the case with some cobalt 

 bands), we must conclude that the absorbing power of a molecule is in- 

 fluenced considerably by its immediate surroundings. As usual, there are 

 at least two possible ways of explaining the increase in the absorption 

 with concentration, when molecules are kept constant. One is to assume 

 the formation of aggregates of molecules, and that the absorbing power of 

 the molecule is increased thereby; the other is to assume the existence of 

 solvates, and that the absorbing power of a molecule decreases with increase 

 in the complexity of the solvate. To decide between these two possible 

 explanations we need only take into account the change in the absorption 

 produced by a rise in the temperature of the solution. This change is the 

 same qualitatively as that produced by increasing the concentration. 

 Molecular aggregates are broken down by rise in temperature, and hence, 

 by the assumption made above as to the effect of aggregates on absorp- 

 tion, this should decrease the absorption instead of increasing it. We must 

 conclude, therefore, that the change in the absorption is not due to the 

 formation of aggregates. 



Solvates are made simpler both by increasing concentration and by 

 rise in temperature; and, accordingly, from the assumption stated above 

 regarding the effect of complexity of solvates on absorption, both changes 

 should produce similar differences in the absorption spectrum, which is in 

 accordance with observed facts. We conclude, therefore, that the ultra- 

 violet absorption of solutions of copper salts is due to the "solvated" mole- 

 cules of the dissolved salt, and that the absorbing power of such molecules 

 is decreased as the complexity of the solvate increases. 



It will be remembered that for equal concentrations the absorption in 

 the region of shorter wave-lengths is least in the aqueous solutions, then 

 increases as we pass from methyl alcohol to ethyl alcohol. In general, 

 also, it may be stated that the change in the absorption with dilution is 

 greatest for the aqueous solution, and then decreases as we pass to methyl 

 and ethyl alcohols. This is just what we should expect, since the power to 



