576 
Mr Purvis, The absorption spectra 
These results, therefore, show that the shift of the band towards 
the red end and also its persistency are not necessarily dependent 
upon the number and mass of the atoms or radical groups intro- 
duced into the nucleus. Their type and spatial relationships or 
orientation must be taken into consideration. 
Baly and Desch ( Astrophysical Journal, vol. xxm. pp 110 — 
127) in a study of the absorption spectra of some aliphatic com- 
pounds exhibiting tautomerism and showing absorption bands, 
state that the oscillation frequency of an absorption band varies 
slightly with the total mass of the molecule, but that it is not 
directly dependent upon the mass of the labile atom. Again, 
they suggest that the persistency of the absorption band is a 
measure of the number of molecules in the changing state at 
a given time. 
The oscillation frequencies of the a- and /3-picolines investi- 
gated by Baker and Baly ( loc . cit .) are not identical ; neither are 
those of the two 3-chloropyridines nor the two amido-compounds 
of this investigation ; and it may be possible to extend the theory 
to closed ring compounds. 
Applied to the compounds of this investigation, it means that 
the number of changes in the 2 : 3 : 5-trichloropyridine is greater 
than in the 3:4: 5-compound. Whilst the number of changes in 
the y-amido-compound is considerably less than in the a-amido- 
compound ; or, as stated before, the spatial positions of the atoms 
or groups of atoms in a ring compound influence the number of 
vibrations in a given time. 
And, first, with regard to the two trichloropyridines. It may 
be that the relative positions of the three chlorine atoms in the 
2:3: 5-compouncl are such that their motions with regard to 
the other atoms and themselves are less hindered than in the 
3:4: 5-compound, with the consequent production of a greater 
number of changes in a given time. A greater absorption of light 
vibrations is thereby produced, and the result is observed in a 
greater persistency of the band. And as regards the two amido- 
compounds, the relative positions of the radical groups compared 
with the other atoms, are such that the number of changes in the 
molecules in a given time in the a-compound is less than in the 
5-chloro-compound, thereby producing less absorption of light, 
and therefore there is a less persistency of the band. And the 
number of changes in the vibrations of the y-compound is so 
much less, that the absorption of light is very considerably 
diminished. 
Now as regards the a- and y-amido-tetrachloropyridines, the 
relative positions of the two NH 3 groups are of considerable 
importance in influencing the absorption of light. In the y-com- 
pound, the orientation of the NH. 2 radical and the other atoms of 
