Light Absorption and Fluorescence. 



13 



the simple case of the binary molecule. With still more 

 complex molecules it becomes necessary to accept the 

 existence of small atomic groupings within the principle 

 atomic groupings. Without going into the resulting 

 system in detail, it may be stated that this will result in 

 the sub-division of the sub-groups into smaller sub-groups. 

 The trace of the ultra-violet ;ibsorption-band of sulphur 

 dioxide is shown in fig. 1, where each sub-group consists 

 of absorption-lines associated in the manner explained with 

 the central line of the sub-group. The next stage in 

 complexity will give a trace similar to that shown in 

 hV 2. It is of considerable interest to note that the 



phosphorescence and absorption-bands shown by certain 

 uranyl compounds exhibit a structure exactly analogous 

 to that shown in fig. 2 *. 



In the above I have only developed the conception of 

 elementary and molecular quanta as far as the explanation 

 of the establishment of that molecular frequency called the 

 infra-red fundamental, and the association of atomic and 

 intra-molecular frequencies with a true molecular fre- 

 quency. There remains to be dealt with the origin of 

 the combination of atoms to form molecules, and also the 



* Nichols and Merritt, Physical Review, vi. p. 630 (1915) ; ix. p. 113 

 (1917). 



