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LIV. Light Absorption and Fluorescence, — Part III. By 

 E. C. 0. Baly, M.Sc, F.R.S., Grant Professor of Inorganic 

 Chemistry in the University of Liverpool*. 



IN the second paper of this series f it was shown that the 

 Bjerrum conception of the structure of the short-wave 

 infra-red absorption bands can also be applied to the 

 ultra-violet and visible absorption and fluorescence bands of 

 organic compounds. Bjerrum J showed that if v x be the 

 frequency of the central line of an absorption-band group in 

 the short-wave region of the infra-red, there will be sym- 

 metrically distributed on each side of this line, absorption 

 lines the frequency of which is given by 



V x ±Vr, 



where v r stands for the rotational frequency of all the mole- 

 cules corresponding to the frequencies of the absorption 

 bands in the long-wave infra-red region. He further showed 

 that on the basis of the energy quantum theory the rotational 

 frequencies must have well defined values given by the 

 expression 



hn 



where I is the moment of inertia, h the Planck constant, and 

 n a series of whole numbers, 1, 2, 3. &c. As stated above, it 

 has been found that the relation also holds good with the 

 ultra-violet and visible absorption and fluorescence bands of 

 substances, for if v be the frequency of the central line of 

 one of these bands, there are found to be symmetrically dis- 

 tributed on each side of this line pairs of absorption or 

 fluorescent lines, as the case may be, the frequencies of 

 which are given by v±.v x , where v x stands for the observed 

 frequencies of the absorption bands in the short-wave infra- 

 red region. It was pointed out that although this relation- 

 ship undoubtedly exists, by no means all the component lines 

 of a given absorption-band group are thus accounted for. 

 The question therefore arises as to whether the complete 

 Bjerrum relation holds, and whether the whole of the com- 

 ponent lines in any one ultra-violet or visible band group 

 can be calculated from the expression 



hn 



v± wv 



* Communicated \>y the Author. 

 + Phil. Maar. xxix. p. 223 (1915). 

 % Nernst, Festschrift, 1912, p. 90. 



