438 Mr Purvis, The absorption spectra of some 
(, loc . cit.), and it will suffice to refer to their paper for the details. 
With regard to the 9-chlorocollidine the examination and com- 
parison of the absorption curve appeared to suggest the great 
probability that the nine chlorine atoms had replaced the hydro- 
gen atoms of the three methyl groups, and that none had entered 
the nucleus. 
The curves of these two compounds have been drawn again in 
order to compare them with the compounds investigated since. 
It has been noticed by Hartley, and by Baker and Baly, that 
the addition of hydrochloric acid to the solutions of pyridine and 
of 2 : 6-lutidine, produces a very great increase in the persistence 
of the absorption bands, in addition to shifts of the band towards 
the red end. 
The present writer has investigated the effect of adding hydro- 
chloric acid to collidine ; and it will be noticed, from an exami- 
nation of the curves, that precisely the same effect was obtained 
as in the case of pyridine and lutidine. That is to say, the 
introduction of hydrochloric acid to the collidine produces a very 
marked increase in the persistence of the absorption band ; whilst, 
at the same time, there is a slight shift of the band towards the 
red end. The persistence is, also, much less than that of either 
lutidine or pyridine hydrochlorides, and corresponds, pari passu, 
to the decreased persistence of the bands of collidine, lutidine and 
pyridine when these substances are examined in solution in the 
basic condition. 
Baker and Baly’s theory is that the addition of the acid satisfies 
the residual affinity of the nitrogen atom in these compounds, 
and thereby removes the restraining influence. Consequently the 
tautomeric process is increased, and the persistence of the band is 
increased. Now, Purvis and Foster (loc. cit.) have shown that, as 
the number of methyl groups in the pyridine nucleus is increased, 
the persistence of the bands is also decreased. That is to say, the 
persistence of the band of collidine (trimethylpyridine) is less 
than that of lutidine (dimethylpyridine), and still less than that 
of pyridine. Further, the increased persistence of the band of 
collidine hydrochloride is less than that of the lutidine hydro- 
chloride, and, therefore, still less than that of pyridine hydro- 
chloride. 
The results, therefore, appear to indicate that, although the 
effect of the addition of acid to collidine is to increase the per- 
sistence of the band, it does not balance the restraining influence 
of the side chains. There appear to be two forces, as it were, 
acting in opposite directions. The vibrations of the ring are 
influenced in one direction, either by increasing the number of 
chlorine atoms in the nucleus, or by the addition of hydrochloric 
acid, by which increased persistence of the band is obtained ; and 
