\l> A STUDY OF THE ABSORPTION SPECTRA. 



and of fluorescence. On such a theory as this it may be supposed that in 

 solutions the absorption took place in those molecules that are undergoing 

 dissociation, or in those ions that are combining to form molecules. In 

 the case of the uranyl nitrate bands it was pointed out that the combined 

 action of water and the N0 3 group had a hypsochromous effect upon the 

 wave-lengths of these bands. That it is due to the combined action of the 

 water and the N0 3 group is shown by the fact that in other solvents the 

 N0 3 group does not have this hypsochromous effect; while in water it is 

 only the nitrate bands that have the smaller wave-lengths. According to 

 the theory of dynamic ionization the absorption of light could take place 

 while the N0 3 groups were near the U0 2 group, so that the periodicity of 

 the absorbed light would be affected. According to this theory, however, 

 the number of ionizations through which a molecule would pass would 

 probably be a function of the concentration; and thus the intensity of the 

 absorption bands should be a function of the concentration. But for ura- 

 nyl nitrate Beer's law holds at least approximately, whereas considerable 

 variations would be expected from the above theory. If the absorption 

 takes place during periods of ionization, and the intensity of the absorption 

 depends only on the number of these ionizations, then the fact that Beer's 

 law holds shows that the number of these ionization phenomena is inde- 

 pendent of the concentration. The fact that the absorption spectrum of 

 uranyl nitrate crystals is very similar to solutions indicates that the uranyl 

 groups that take part in absorption are about as closely united with water 

 and the N0 3 group in solution as they are in the solid. 



It may, however, be said in general, that the anions of the various col- 

 ored salts and in practically all cases it is the anion that exhibits banded 

 absorption play a much less important role in modifying the spectra than 

 the solvent. Different salts of the same anion in the same solvent usually 

 have the same absorption spectra. On the other hand, the absorption 

 spectra of the powdered salts themselves may be very different. This fact 

 shows that the solvent plays a most important part in the absorption, and it 

 seems highly probable that in a large number of cases there is an " atmosphere " 

 of the solvent molecules about the colored absorber. In a word, there is solva- 

 tion. However, in the case of the nitric oxide 1 spectrum it seems possible 

 to have the gas existing in solution and at the same time having its ab- 

 sorption spectra unaffected. The nitric oxide spectrum only occurs under 

 very special conditions, and has not thus far been obtained for solutions 

 of the gas, but only when some acid is added to nitric acid or a nitrate. 

 It would seem probable that in this case no chemical reaction or 

 " atmosphere " of the solvent existed. 



Whereas the absorption of different salts of the same colored anion is 

 in general very similar, on the other hand, the absorption spectra of the 

 same salt in different solvents are often very different indeed. Formerly this 

 effect of the solvent was thought to be due to a difference in the value of the 

 dielectric constant, but Jones and Anderson have pointed out that the 

 most probable cause is the formation of solvates, or more or less stable com- 



'Phys. Rev., 30, 279 (1910). 



