180 DISCUSSION OF EVIDENCE. 



much finer and sharper than they are in isobutyl alcohol. Further, 

 the bands of neodymium chloride in isobutyl alcohol have slightly 

 greater wave-lengths than in butyl alcohol. 



To eliminate the possibility of the effect of the solvent on absorption 

 spectra being due to anything inherent in the nature of neodymium 

 chloride, the nitrate of neodymium was studied in the same way as 

 the chloride. 



The absorption spectra of neodymium nitrate in water, in methyl 

 alcohol, in ethyl alcohol, in mixtures of these alcohols and water, in 

 propyl and isopropyl alcohols, in butyl and isobutyl alcohols, in acetone 

 and in mixtures of acetone and water, in ethyl ester and in formamide, 

 were carefully photographed and studied. Results are given below in 

 the case of neodymium nitrate only for the a bands. 



a Bands. 



In water. Practically the same as the bands of neodymium chloride, but 

 the bands of the nitrate are broader and hazier than those of the chloride. 



In methyl and ethyl alcohols. There are only two bands in the a group, 

 X 3465 and X 3545. 



In propyl alcohol. XX 3455, 3500, and 3585. 



In isopropyl alcohol. XX 3460, 3505, and 3535. 



In butyl alcohol X\ 3450, 3500, and 3540. 



In isobutyl alcohol. Ultraviolet absorption was so great that on the plate 

 taken the a group did not appear. The absorption in general is the same as 

 that of the chloride in this alcohol. 



In acetone. XX 3475, and 3555. 



In ethyl ester. XX 3455, 3500, and 3540. 



The other groups of absorption bands of neodymium nitrate in the 

 different solvents show differences in the wave-lengths comparable with 

 the above; but these results suffice to show the effect of the solvent 

 on the power of neodymium nitrate to absorb light. 



The above is strong evidence that the solvent plays an important 

 part in the absorption of light by substances dissolved in it. When we 

 take into account the number of salts studied and the number of 

 solvents employed, the evidence is little short of proof. The only 

 reasonable question is, How are we to interpret these facts? Before 

 attempting to answer this question we should take into account also 

 the following fact : A salt dissolved in a given solvent is characterized 

 by a definite absorption spectrum. When a salt is dissolved in mix- 

 tures of varying proportions of two solvents, only two definite absorption 

 spectra appear, one being characteristic of each solvent. One spectrum 

 does not gradually change into the other as the composition of the mixed 

 solvent changes, but only the relative intensities of the two spectra 

 vary. Starting with that mixture of the two solvents in which both 

 of the spectra are equally intense, if we diminish the amount of a 

 relative to b, the spectrum corresponding to a becomes feebler and 

 feebler, and the spectrum corresponding to b more and more intense. 



