SECTIONAL TRANSACTIONS.— B. 341 



future. Moreover, the practice of electroplating will no doubt be further 

 modified and extended as a result of new advances in pure science, and the 

 range of application of electrodeposited coatings widened by new develop- 

 ments in other industries. 



In connection with this discussion an Exhibit of Electroplating, 

 specially arranged by the Electrodepositors' Technical Society, was 

 on view in the Chemistry Laboratory of the Grammar School, adjacent 

 to Section B meeting-room, from Thursday, September 10, to Tuesday, 

 September 15, inclusive. 



Afternoon. 

 Excursion to Standfast Dyers and Printers, Ltd., Lancaster. 



Monday, September 14. 



Discussion on Photochemistry. 



Dr. R. G. W. Norrish, F.R.S.— Introduction (10. o). 



Mr. H. W. Thompson. — Molecular spectra as a guide to photochemical 

 reaction (10.30). 



The earlier measurements on photochemical reactions were made largely 

 to determine the quantum efficiencies of the reactions from the standpoint 

 of the Einstein Law. The development of the theory of reaction chains 

 from the study of the effect of inhibitors, the high values of the quantum 

 yields, and other considerations, made it clear that there are two essential 

 parts to a photochemical change : (1) the primary process ; (2) secondary 

 processes, which are quite independent of the primary one. It is with the 

 so-called ' primary processes ' that spectroscopic considerations are mainly 

 concerned, for it is clear that the nature of the absorption spectrum of the 

 reacting substance must be a guide to the manner in which the light energy 

 is absorbed. This correlation of absorption spectrum data with photo- 

 chemical measurements has perhaps been the most significant advance in 

 the subject in recent years. 



Briefly, it can be said that the main problem is to decide whether the 

 primary process involves mere excitation, or one-act dissociation. The 

 spectral evidence often makes a decision between the two alternatives 

 possible, although there is in some cases, especially with polyatomic mole- 

 cules, some ambiguity. Spectral evidence also makes it frequently possible 

 to determine the products of a primary one-act dissociation, but the limita- 

 tions of this raises many crucial questions. 



In the present paper the nature of the energy levels of a diatomic molecule 

 and the theory of the structure of an ideal absorption spectrum are outlined. 

 The significance of potential energy curves is discussed. Three types of 

 absorption spectrum are found in practice, (a) those which are fine-structured 

 or discrete throughout, (b) those which are entirely continuous, and (c) 

 those which show either diffuseness throughout or have both discrete and 

 diffuse regions. The more important examples of each type are given, and 

 their interpretation in connection with typical photochemical processes is 

 discussed. In particular, the class of ' predissociation spectra ' is described, 

 and inferences drawn from them are explained. 



