420 Prof. Baly and Mr. Tryhorn on Light 



accurate measurement of the central wave-number of the 

 less refrangible ultra-violet band, and since, further, this 

 must be an exact multiple of the true fundamental wave- 

 number, it follows that the true fundamental wave-number 



or cry 



must be —. p= — = 326*09, because 11 is the only integral 



divisor which gives a value a little larger than 323. It 

 follows, therefore, that the wave-number of the fundamental 

 infra-red band of pyridine vapour should be 1/326*09 = 

 3*066 yu., and also that the central wave-number of the more 

 refrangible pyridine vapour band should be 12x326*09 = 

 3913'08. As was pointed out above, the values given in 

 Table I. show that the central wave-number of the absorption 

 band at infinite dilution is a little greater than 3910, and it 

 is therefore evident that with increasing dilution the centre 

 of the absorption band approaches the true value for the 

 vapour. The measurements in Table I. show that the 

 maximum shift occurs in 10 N solution, which almost exactly 

 corresponds to the molecular ratio of C 5 H 5 N to H 2 of 1:1; 

 and therefore it may be concluded that the maximum effect 

 of the water, as far as altering the free vibration period of 

 pyridine is concerned, occurs when the solvent and solute 

 are in the proportions agreeing with the monohydrate 

 C 5 H 5 N,H 2 0, and that the influence of the water decreases 

 with dilution until, finally, the free vibration period becomes 

 the same as that of pyridine vapour. 



It is also evident that we now have an explanation of the 

 variation in position of an absorption band when a substance 

 is examined in solution, namely, that it is due to a combina- 

 tion or a tendency to a combination of the substance with 

 the solvent. Moreover, it also follows that the fundamental 

 infra-red band must also shift with solution ; and in the 

 fourth column of Table I. are given the wave-numbers of 

 this infra-red band at the various dilutions, i. e. l/\ -r- 12. 

 The wave-lengths of the infra-red bands are given in the 

 fifth column of Table I. For obvious reasons it is impos- 

 sible to measure the infra-red band for dilute solutions, but 

 from some observations made in these laboratories it is clear 

 that this band shifts in a manner comparable with that shown 

 in Table I. The wave-numbers of the infra-red band have 

 been plotted against the logarithms of the corresponding- 

 volumes of solution, and the curve obtained is shown in 

 fig. 1, curve I. 



From the value given in Table I. it is clear that the 

 fundamental infra-red band of pure homogeneous liquid 

 pyridine should lie at 3*096 /z. The absorption at this 



