SECTION B.— CHEMISTRY. 



PHOSPHORESCENCE, FLUORESCENCE 

 AND CHEMICAL REACTION. 



ADDRESS BY 



PROF. E. C. C. BALY, C.B.E.. M.Sc, F.R.S., 



PRESIDENT OF THE SECT ON. 



The phenomena associated with chemical reaction, and in particular the 

 mechanism of chemical change, form a subject of peculiar interest. The 

 story of the development of ideas from the birth of modern chemistry to 

 the present day is one which to my mind forms the most attractive chapter 

 in the history of our science. It may be that to some of those who earn 

 undying fame by the determination of the constitution of most wondrously 

 complex molecules, to some of those who go down to posterity as masters 

 of synthesis and wizards of organic method, it may be that to these this 

 chapter presents an interest that is languid. On the other hand there 

 are many to whom it makes a great appeal because the subject matter is 

 the fundamental basis of chemical knowledge. I confess my own 

 allegiance with the latter, but it is in a very humble spirit that I venture 

 to speak upon this subject. I do so not with any confident assurance of 

 being able to put forward a theory of chemical reaction which will embrace 

 all the known facts and embody all the views that have from time to time 

 been enunciated, but rather in the hopes of collecting together a number 

 of observations which have been made in fields allied to chemistry and 

 appear to be worthy of consideration by those who seek to find an explana- 

 tion of the mechanism of chemical reaction. 



The allied fields to which I refer are those of phosphorescence, 

 fluorescence and absorption spectra, fields which have been enriched by 

 observations of high accuracy. These observations are of special signifi- 

 cance in that they are concerned with the physical properties of molecules 

 in contradistinction to those of atoms. The phenomena of chemical 

 reaction are essentially associated with the absorption and radiation of 

 energy, and it thus seems somewhat strange that little attempt has hitherto 

 been made in considering the mechanism of reaction to invoke aid from 

 the many investigations in these allied fields which obviously deal with 

 the energy changes undergone by molecules. It will be my endeavour 

 to show that the evidence that has been obtained from the study of 

 luminescence and absorption spectra has a very direct bearing on the 

 phenomena of chemical reaction,- and that the hypothesis of activated 

 molecules which forms the basis of the modern theories of the latter can 

 be rigidly tested and examined by the former. 



One of the most important theories brought forward during recent 

 years is that known as the radiation hypothesis, which was developed 



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