418 



Prof. Baly and Mr. Tryhorn on Light 



-contain the wave-lengths and wave-numbers respectively of 

 the centre of the absorption band. 



Table I. 



V. 



X. 



l/\. 



Factors. 



Infra-red Band. 



■08065 



2580 



3876 



12x323-0 



3096 n 



o-i 



2612 



3828 



12x319-0 



3-135 



0-160 



2595 



3S53 



]2x321-l 



3-114 



10-0 



2572 



3888 



12x324-00 



3-086 



12-0 



2570 



3891 



12x324-25 



3-084 



500 



2562 



3903 



12x325-25 



3075 



25000 



2558 



3910 



12x325-83 



3-069 



The first value (V = 0'08065) refers to pure liquid pyridine, 

 and, as can be seen, the band shifts towards the red on the 

 addition of a small quantity of solvent and then back towards 

 the ultra-violet as the concentration of the solution decreases. 

 It is obvious from the values given in the table that the wave- 

 number of the centre of the band at infinite dilution must 

 be a little greater than 1/X = 3910. Now the absorption of 

 the vapour of pyridine has been observed by Purvis *, who 

 has measured the wave-lengths of fifty-three absorption lines 

 composing the single band group. The wave-numbers of 

 these may be arranged symmetrically around the strong line 

 with wave-length \ = 2789 as centre. Then, as shown in 

 the previous paper f, the differences between the wave- 

 numbers of the central line and the wave-numbers of the 

 other lines must equal the wave-numbers of the infra-red 

 bands of pyridine. As the infra-red spectrum of pyridine 

 has only been observed between the limits 3 /a and 13 fi, it 

 is only possible to determine whether the relation holds 

 good between these limits. It is, however, manifest that 

 the relation holds good from the values given in Table II. 

 The first and second columns contain the wave-lengths of 

 the absorption lines as measured by Purvis and their wave- 

 numbers (1/X). The third column shows the wave-number 

 differences between each line and the central line ; while the 

 fourth column contains the means of the two values where 

 such exist. The reciprocals of these differences or the wave- 

 lengths of these infra-red bands are given in the fifth column. 



* Trans. Chem. Soc. xcvii. p. 692 (1910). 

 t Phil. Mag. xxix. p. 223 (1915). 



