Chemical Constitution of Saline Solutions. 298 
acetate and carbonate yielding different ions from the chloride and nitrate, or, 
more precisely, they are at a loss to account for the neodym ion exhibiting a 
different spectrum when disengaged from these different salts. 
From the point of view, which I have held for some years past, there is no 
difficulty whatever in accounting for these differences, for it is perfectly clear that 
the absorption spectra of such solutions are not derived from the metal alone or 
from the metallic oxide, but they are absolutely the property of the salt molecule 
as a whole, as I have shown to be the case with various organic substances, hydro- 
carbons, phenols, acids, bases, salts, and dyes, with didymium salts, and other 
metallic compounds. These absorption bands are as essential a characteristic of 
the respective salts as the six bands in the spectrum of benzene or the two or three 
bands in that of cymene. They follow the same rule with regard to the weighting of 
the molecule, and also as regards dilution and the gradual extinction of the bands. 
While reserving any opinion upon the question of whether electrolytic dissociation 
of salts in solution necessarily implies the complete decomposition of the salts into 
separate ions and a corresponding change in the spectra of the salts, I may remark 
that, at the present time, there is nothing in their spectra which can be regarded 
direct and conclusive evidence of this. The experiments of Ostwald,* carried 
out on a variety of permanganates, really tell us nothing beyond the fact that great 
dilution caused the solutions to exhibit three absorption bands, which appeared to 
be practically identical, though one only of them was measured. The photographs 
show that the bands were vague, and this is, in fact, the character of all the seven 
bands, which are situated in the green and yellow region, where dispersion is 
small and the alteration in the wave-lengths of the absorbed rays is not easily 
measurable. 
Ostwald, in fact, merely gives their positions, and not their wave-lengths. By 
increasing the dispersion so as to make differences in wave-length appreciable, the 
bands become so vague that there is nothing which can be measured with any 
certainty. It occurred to me, on examining the experimental evidence afforded 
by the spectra of these permanganates, that the differences in atomic weight of the 
basic elements forming the electro-positive ions of the salts were not sufficiently 
great to cause any considerable ‘retardation of the intra-molecular vibrations 
which could be rendered visible at the red end of the spectrum ; and that in order 
to test this matter, substances should be chosen which exhibit sharply defined bands 
either in the blue, violet, or ultra-violet, where the wave-lengths are shorter and 
the dispersion greater without loss of definition. In fact, ultra-violet absorption 
spectra are much more sharply defined than those in the regions of larger wave- 
length in the direction of the red. 
From the salts of didymium, chloride, sulphate, nitrate, and acetate, and also 
* Zeitsch. fiir physik. Chemie. 
TRANS. ROY. DUB. SOC., N.S. VOL. VII., PART VIII. 28 
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