solid tetramethylpicene and of its solutions. 83 



resultant absorption spectrum and they were compared with the 

 absorption spectra of the benzene and alcoholic solutions. It was 

 found that in the three series of experiments the substance showed 

 three absorption bands, but that the relative position of the bands 

 varied. The bands of the solid were moved more towards the red 

 end of the spectrum than those of the alcoholic solution, whilst 

 their positions in the benzene solution were between those of the 

 solid and the alcoholic solution. In order to compare the results 

 the following table gives the numbers of the three bands in terms 

 of the mean oscillation frequencies : 



C^U,,, solid 2264 2405 2537 



i\^/1000 benzene solution 2290 2424 2584 

 i\^/1000 alcoholic „ 2304 2441 2590 



The numbers for the benzene solution are extracted from the 

 before-mentioned paper by Homer and Purvis (loc. cit). And 

 comparing the positions where general absorption begins the 

 numbers are: 



C26H22, solid 2182 



benzene solution 2212 

 alcoholic „ 2247 



It is evident from these numbers that there was a shift in the 

 position of the bands and of the general absorption towards the 

 red end of the spectrum according to the density of the medium, 

 whilst in the solid state the shift was more marked still. 



The rate of vibration of the molecules of the solute must be 

 affected by the molecules of the solvent, that is to say the more 

 dense the medium the greater damping effect will it have on the 

 rate of vibration of the dissolved molecules. 



Now in the solid state the mean free path of the molecules is 

 more restricted than when they are distributed throughout some 

 solvent, therefore the rate of vibration of the molecules of the 

 substance in the solid state should be slower than when in 

 solution. 



Assuming that the general absorption is due to the vibrations 

 of the molecule and that the selective absorption is caused by 

 intra-molecular vibrations of the atoms which are also affected by 

 the molecular vibrations, then conditions which tend to damp the 

 rate of the molecular vibrations, that is, which cause a shift in the 

 general absorption will also cause a corresponding shift in the 

 selective absorption of the substance. 



The results obtained are in accordance with this view, for a 

 comparison of the positions of the general and selective absorption 

 of the hydrocarbon in alcoholic and benzene solutions and in the 

 solid state shows that for benzene solutions there is a shift towards 



