436 Mr Purvis, The absorption spectra of some 
The positions of the heads of the bands, expressed in oscillation 
frequencies, are cited in the following table : 
3 : 5-Dichloropyridine 3700 (Purvis) 
3:4: 5-Trichloropyridine 3650 (Baker and Baly) 
2:3:4: 5 -Tetrachloro pyridine 3500 „ „ „ 
Pentachloropyridine 3400 „ ,, „ 
Oscillation frequencies 
The persistence of the band of the dichloro pyridine compound 
is certainly less than that of the trichloropyridine, although the 
difference is not so well marked as appear to be the differences 
between the tri-, tetra-, and pentachloropyridines. 
But the results obtained, from a study of the dichloropyridine, 
confirm generally the previous observations, namely, that the in- 
troduction of chlorine into the pyridine nucleus produces a gradual 
shift of the absorption band towards the red end of the spectrum ; 
and the greater the number of chlorine atoms the greater the shift. 
And, also, that a gradually increasing number of chlorine atoms 
entering into the ring, produces a gradual increase in the per- 
sistence of the band. 
Collidine hydrochloride. To turn now to the collidine com- 
pounds, the absorption bands of collidine and 9-chlorocollidine 
have already been described and discussed by Purvis and Foster 
