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XXVI. Light Absorption and Fluorescence. — Part II. By 

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

 Chemistry in the University of Liverpool*. 



IN a previous paper f the existence of constant differences 

 between the frequencies of absorption bands and fluores- 

 cence bands exhibited by many organic compounds was dealt 

 with, and I showed that it was possible to account for these 

 on the basis of the energy quantum theory. Several com- 

 pounds are known each of which shows three absorption 

 bands and at least one fluorescence band, and in these cases 

 there exists a constant difference between the frequencies of 

 the centres of these bands. I pointed out that the absorbed 

 energy must be emitted again at some frequency which is 

 characteristic of the molecular system, which frequency may 

 be either in the visible spectrum, when we have fluorescence, 

 or in the infra-red. Then on the energy quantum theory a 

 single quantum absorbed at the higher frequency must be 

 given out as a whole number of quanta at the lower frequency. 

 If, therefore, v z be the frequency of the characteristic vibra- 

 tion in the infra-red, the energy can only be absorbed at the 

 frequencies z^, v^ v^ &c, where v l9 v 2 , p 3 , &c. are successive 

 multiples of v x . Obviously, therefore, there must be a 

 constant difference (vx) between the frequencies v l5 v 2 , v 3 , &c, 

 and this difference must equal the frequency of the infra-red 

 vibration. 



Since that paper was published a number of compounds 

 have been accurately examined, and the above relation is 

 found to hold good. These results will it is hoped be pub- 

 lished in a separate communication. I omitted in the previous 

 paper to point out that the existence of these constant 

 frequency differences has also been noted by v. Kowalski 

 in a paper dealing with the phosphorescence of a number of 

 organic compounds J. 



Two years ago a paper was published by Bjerrum §, in 

 which he dealt with the short-wave infra-red absorption 

 bands of certain compounds. He points out that if v be the 

 characteristic frequency of the atoms in a given molecule, 

 then, if v r be the frequency of rotation of the molecules, 

 three bands should be shown in the neighbourhood of the 

 band v. The frequencies of these will be v—v r , v. v + v r 

 respectively. Since the central vibration is pure it will 



* Communicated by the Author. 

 t Phil. Mag-, xxvii! p. 632 (1914). 

 t PJu/s. Zeit. xii. 1911, p. 950. 

 § Nernst, Festschrift, 191 L>, p. 90. 



