28 Prof. E. C. C. Baly on 



quanta absorbed and the number of molecules which react 

 in thermal reactions where the absorption of energy takes 

 place at frequencies in the infra-red. In any exothermic 

 reaction there will be a discrepancy in the sense that more 

 molecules will react than are calculated on the basis of 

 the quantum theory. Lewis* has drawn attention to the 

 serious discrepancy which occurs in the thermal decom- 

 position of phosphine, the velocity of the reaction being 

 enormously greater than that calculated on the quantum 

 theory. The decomposition of phosphine to give hydrogen 

 and phosphorus is an exothermic reaction f; so that, if the 

 energy radiated during the decomposition is re-absorbed by 

 fresh molecules of phosphine the observed reaction velocity 

 will be greater than that calculated on the basis of a direct 

 relation between the energy absorbed from external sources 

 and reaction velocity. The divergence will be enormous if 

 the proportion of internally radiated energy that is re- 

 absorbed is large. I venture to think that in this is to be 

 found the explanation for which Lewis asks. 



In general, it would seem probable that the proportion of 

 internally radiated energy that is re-absorbed by the system 

 will depend on the temperature and pressure. For example, 

 it would be natural to expect that this proportion would be 

 increased if the molecular concentration of the reacting 

 system is increased. Without doubt this explains the much 

 greater divergence from Einstein's law that is found in the 

 case of a photographic plate than in the case of photo- 

 chemical reactions in dilute solutions. Thus Henri and 

 Wurmser found that with 0*037 normal solution of hydrogen 

 peroxide 180 molecules were decomposed as the result of the 

 absorption of one quantum at the phase frequency. With 

 a normal solution of acetone 1360 molecules were decom- 

 posed for every phase quantum absorbed. On the other 

 hand, these authors point out that Leimbach's measurements 

 with a photographic plate lead to the conclusion that five 

 million molecules of silver bromide are reduced to silver for 

 every quantum absorbed at the phase frequency of silver 

 bromide. These figures show clearly the increase in the 

 divergence from the Einstein law that results from the 

 increase of molecular concentration. This agrees with 

 the view that the divergence is due to the re-absorption of 

 the internally radiated energy. 



There are obviously many directions in which this mole- 

 cular phasy hypothesis can be quantitatively applied, and 



* Phil. Mag. xxxix. p. 26 (1920). 



t P. Lemoult, Comptes Rend. cxlv. p. 874 (1907) 



