the Rotation-Energy of Molecules, 7& 



according to Bjerrum, emission and absorption may be 

 expected not only for v and v, but also for v + v. If, as 

 has been assumed hitherto, the rotation-frequencies of the 

 molecules are distributed according to Maxwell's law of 

 distribution, then the short-wave, ultra-red absorption-bands 

 might be expected to consist of a sharp maximum for v and 

 two extended absorption-areas with maxima for v + v, when 

 z7 is the most probable rotation-frequency. For the very long- 

 wave-lengths the strongest absorption can be expected in the 

 case of the wave-length that corresponds to v, and this long- 

 wave absorption can thus be calculated as soon as the two 

 maxima v ±v are known. 



The latest investigations into the subject of ultra-red 

 absorption have corroborated this view of Bjerrum's. It is 

 true that the three maxima anticipated by Bjerrum have not 

 been found, as it has been shown that, in general, the bands 

 have a tendency to dissolve into double bands *, but this is a 

 natural consequence of the circumstance that in this part of 

 the spectrum it has not been possible to work with sufficiently 

 great dispersion, but at the same time we always measure a 

 relatively considerable part of the spectrum — in the most 

 favourable case 10-20/jl/jl* If the absorption for v consists 

 of a narrow line it can, on that account, only contribute to a 

 very small extent to the absorption-value found. Besides, it 

 must be considered as very uncertain — at least for diatomic 

 gases — whether there is any absorption at all for v . 



Fig. 1 1 gives the strongest absorption-bands for carbon 

 monoxide, nitrous oxide, and carbon dioxide, and similar 

 absorption-curves have been found for a whole series of other 

 gases. If we calculate from these bands the absorption in 

 the case of long wave-lengths, we shall find fairly close 

 correspondence with the results obtained directly by Rubens 

 and v. Wartenberg J. 



Strong support for Bjerrum's view as to the origin of the 

 ultra-red absorption-band is afforded, too, by the influence of 

 temperature. According to the kinetic theory of gases the 

 most probable rotation-frequency ought to increase pro- 

 portionally with the square root of the absolute temperature 

 (T) ; hence the frequency-difference between the band's two 

 maxima also ought to increase proportionally with -v^T, and 



\/T . .? A 2 ought to remain constant, when \j and \ 2 are 



the wave-lengths for the two maxima. 



* W. Burmeister, Verh. d. D.phys. Ges. xv. p. 589 (1913). 

 t E. v. Bahr, Verh. d. D.phys. Ges. xv. p. 710 (1913). 

 1 L.c. 



