Absolution and Fluorescence. 425 



to be independent of a knowledge of the infra-red spectrum, 

 a matter of great importance in view of the fact that the 

 infra-red absorption spectra have been observed of only 

 relatively few substances. This is partly due to the fact 

 that the experimental technique is relatively difficult, but 

 also to the fact that no investigations have been made of the 

 absorption of solutions in the infra-red region. The result 

 has been that solids of high melting-point have not been 

 examined. In view of what was said above about solutions, 

 it is to be hoped that investigations of their infra-red 

 absorption will be undertaken, and, indeed, we ourselves 

 look forward, when more pressing matters have been disposed 

 of, to carrying out some of the work in this field. 



In the absence of knowledge of the infra-red spectra, it is 

 possible, as stated above, to argue from the constant wave- 

 number differences between the bands in the visible and 

 ultra-violet regions. The existence of these constant differ- 

 ences has now been proved for so many compounds, that the 

 relationship may finally be accepted as absolute. It therefore 

 becomes possible to use the converse argument, namely that, 

 if a substance exhibits different absorption bands in different 

 solvents and if the constant difference relation holds good 

 between the various absorption bands, the change in absorp- 

 tion in passing from one solvent to another cannot be 

 interpreted as an indication that the substance has changed 

 its constitution. It is, however, believed by many chemists 

 that a change in absorption produced by a change in solvent 

 always means a change in the primary structure of the 

 compound in the chemical sense, the belief being apparently 

 based upon the conception that a particular absorption curve 

 must be characteristic of a definite molecular structure, and 

 that it is not possible to call into play any free period of 

 vibration which previously was latent. As may readily be 

 understood, it was not possible to prove the existence of the 

 constant difference relation for solutions until the variation 

 in the position of the absorption bands with concentration 

 had been worked out. Now that this variation is understood, 

 it is possible to apply the argument to the problems of 

 chemical constitution. This has now been done for the 

 phenols and nitrophenols with results that are eminently 

 satisfactory, but a description of this work must be reserved 

 for a further paper. 



The second point to be discussed is the variation in the 

 absorptive power with concentration of the absorbing com- 

 pound in a given solvent. 



