574 Prof. E. 0. 0. Baly on Light 



differences between the sub-groups in the solution phos- 

 phorescence band of naphthalene. According to the theory 

 of molecular rotational frequencies, this frequency difference 

 of 1*4136 x 10 13 should be the rotational frequency of 

 naphthalene, and I therefore ventured to prophesy that 

 this compound should exhibit a series of absorption-bands 

 in the infra-red with frequencies given by 1*4136 x m x 10 13 r 

 where m = l, 2, 3, etc. The infra-red absorption-spectra of 

 naphthalene had not then been observed, but it has recently 

 been examined by Stang * with the specific aim of putting' 

 my prophecy to the test of experiment, and he found 

 no evidence of absorption-bands at those positions. The 

 experience gained from sulphur dioxide, and now from 

 water, shows that the frequency differences exhibited 

 between the compound sub-groups of an absorption-band 

 are not molecular but infra-molecular. It does not neces- 

 sarily follow therefore that they will be exhibited as 

 infra-red frequencies of naphthalene itself, but we are 

 brought to the conclusion that they are characteristic of 

 a definite atomic grouping present in naphthalene. Now 

 the two principal atomic groupings in naphthalene are the 

 benzene nucleus and the define linking. The intra- 

 molecular origin of the frequency 1*4136 X 10 13 would 

 lead to the belief that this number is characteristic of — 

 and, indeed, is the true molecular frequency of — either 

 benzene or the olefines. In other words, benzene or the 

 olefines should exhibit absorption-bands the central lines 

 of which have the frequencies 1*4136 X mX 10 13 or the 

 wave-lengths 21*22 ^ 10*61 /n, 7*07/*, 5'31/^ 4*24 ft, etc. 

 Moreover, of these the intensity of absorption will be a 

 maximum at \ = 21*22 and fall with decreasing wave- 

 length. 



Now the absorption-spectrum of benzene shows important 

 bands at 9*78 /x, 6*75^,5*5^, 3*25 /jl, and obviously therefore 

 the above frequency is not associated with the atomic 

 grouping of benzene. On the other hand, this frequency 

 is absolutely characteristic of the olefines — a series of 

 hydrocarbons which are almost identical in their absorptive 

 power in the infra-red. Coblentz has investigated the 

 region between 13 /a and 2 /jl, and he found bands with 

 centres at 10*5 /x, 6*98 /jl, 5*30 ^, and 4*32 /x. These four 

 bands are the only ones which the olefines show between 

 13 /jl and 4 /i, and the amount of light absorbed at these 

 maxima with the layer of olefine used was 98, 81, 47, 

 and 14 per cent, respectively. Since Coblentz did not 

 * Stang, Phys. Few vol. ix. p. 542 (1917). 



