90 Mr PiD-ms, The absorption f^pccfra of mesitylene, etc. 



and the explanation was that such an introduction reduced the 



residual affinity of the nitrogen atom, and thereby allowed greater 

 freedom of vibration or pulsation of the ring. 



The author has pointed out in previous communications 

 {Proc. Gamh. Phil. Soc. vol. xiv. (1908), pp. 881, 435 and 568) 

 that the persistency of the band in pyridine compounds is con- 

 ditioned to a considerable extent by the relative or spatial 

 positions of the introduced chlorine atoms as well as by their 

 introduction either in the nucleus or the side chains. 



Now the increased persistence of this strong band of trichloro- 

 mesitylene is very similar to that of the chlorine compoiuids of 

 pyridine and several of its methyl derivatives, where the chlorine 

 atoms had entered the nucleus, and, unless we assume some 

 changing valency of the atoms of the benzene ring, it does not 

 seem easy to correlate the two series of phenomena. 



But before any definite conclusion can be drawn from these 

 observations on mesitylene and trichloromesitylene, it will be 

 necessary to examine the absorption spectra of other chlorine 

 compounds of the former substance, both when the chlorine atom 

 or atoms are in the nucleus and when they are in the side chains. 



All that can be said at the present stage of the investigation 

 is that (1) the strong absorption band of the trichloromesitylene is 

 shifted more towards the red end of the spectrum than the corre- 

 sponding band of mesitylene, and that (2) the persistence of the 

 band of the former compound is much greater than that of the 

 mesitylene. 



Mr W. H. Foster, of St John's College, was good enough to 

 make up the solutions of the two substances examined. 



