of some compounds of pyridine. 
577 
the molecule, particularly the relative position of the nitrogen 
atom of the nucleus and the NH 2 radical, may hinder the vibrations 
so that the number of vibrations is considerably less than in the 
a-com pound, with a consequent less absorption of light, and, 
therefore, a very great decrease in the persistency of the band. 
On the other hand, it is necessary to point out that the persistency 
of the band and its shift may also be connected and explained, in 
a similar manner as in the case of the trichloropyridines (p. 572). 
General Conclusions. 
But whatever be the physical explanation of the differences in 
the shifts and persistencies of the absorption bands of these allo- 
tropic compounds obtained from pyridine, the absorption curves 
indicate that : 
(1) The relative positions of the absorption bands are in- 
fluenced by the type and the spatial positions of the atoms or 
groups of atoms introduced into the nucleus, and 
(2) The persistencies of the bands are also influenced by the 
type and the spatial positions of the atoms or groups of atoms 
introduced into the nucleus. 
I wish to thank Dr Sell for his great kindness in giving me 
specimens of the pure substances used in this investigation. 
