Although measurements on the ultra-violet absorption spectra of polyatomic molecules have rapidly multiplied in recent years, probably in no case has the structure of the entire spectrum been satisfactorily and completely interpreted. From the chemical point of view, investigations have been mainly directed to the study of “predissociation” processes and their correlation with the primary processes of photo chemical change, whilst in addition some knowledge has been gained in regard to the products of photodissociation and energies of linkage. A more careful examination of the matter has now shown that the inferences to be drawn from predissociation phenomena must be made with care, and in many cases additional measurements—for example of fluorescence or of quantum efficiencies—have to be made before the interpretations become unambiguous. From the physical standpoint, only a few band systems have been analysed in detail (
e
.
g
., ClO
2
, Urey and Johnston*; SO
2
, Watson and Parker,) and even in these the interpretations given may not be accurate. One aspect of the matter which has not yet received much attention, is the nature and type of the vibrations excited in polyatomic molecules. This may prove to be of considerable importance in connection with chemical kinetics. The chief difficulty in the analysis of the spectra of polyatomic molecules usually arises from their complexity, whilst the frequent occurrence of purely continuous spectra which may or may not overlap band systems often makes it impossible to derive much knowledge of the molecular excited states. In such cases as the latter, it may be that further in the ultra-violet,
i
.
e
., in the Schumann region, discrete band systems may lead to knowledge of higher electronic states, but this region has so far been little explored. Herzberg and Teller* have recently attempted to construct selection rules for electronic and vibrational transitions in polyatomic molecules, but even when such rules as these are applied and when the infra-red and Raman frequencies are well known, the analysis of most band systems still remains very difficult.
The absorption spectra of some polyatomic molecules containing methyl and ethyl radicals
