I 



B.— CHEMISTRY. 47 



of the reactaut by the absorption of one quantum of energy, which has 

 been called the critical quantum of activation. Evidence gained from the 

 experimental investigation of the phenomena of photoluminescence gives 

 strong support to the reality of this critical quantum of activation, but 

 entirely disposes of the possibility of a molecule gaining this quantum by 

 a single absorption process. The photochemical activation of molecules 

 has been discussed in the light of the evidence gained from the fields of 

 photoluminescence and absorption spectra and the destination of the 

 whole of the energy gained by a molecule when it absorbs its photo- 

 chemical quantum has been traced. Lastly, the connection between 

 the observed heat of a reaction and the critical increments of activation, 

 derived by the radiation hypothesis, has been extended to the photo- 

 chemical quanta, which is an advantage, since the photochemical fre- 

 quencies can be directly observed by spectroscopic methods. It may 

 even be considered that the exhumation of the radiation hypothesis has 

 been partly justified. 



There is no doubt, however, that this partial justification raises the 

 question of thermal reactions in a form which is even more acute than was 

 the case at the inception of the radiation hypothesis. The inability of a 

 molecule to gain its critical quantum of activation by means of a single 

 absorption process has been demonstrated in a far wider field than was 

 covered by the experiments of Lindemann and G. N. Lewis, which as a 

 matter of fact were devised ad hoc. Unless some mechanism exists whereby 

 a molecule can gain its critical quantum of activation from a source of 

 infra-red radiation, photochemical activation must be viewed as an 

 abnormal event and the exhumed radiation hypothesis must be re-interred 

 at once and for all time. It is only fair to ask that the question of thermal 

 reaction be approached and discussed entirely without prejudice, and this 

 is all the more necessary because it has generally been felt that not a 

 single hope remained for the Itypotliesis and men's thoughts have turned 

 to activation by collision with a tendency to exclude any other possibility. 



I have been led to re-open this question by some recent observations 

 which appear to throw new light on the problem. These observations 

 encourage me to suggest a possible mechanism of activation by infra-red 

 radiation. Some justification may be found in the fact that it offers an 

 explanation of many of the difficulties that have been met with in inter- 

 preting the phenomena observed in absorption spectra. 



Mr. Hood at Liverpool has succeeded in determining the temperature 

 coefficient of the reaction whereby carbohydrates are photosynthesised 

 from carbonic acid in the presence of pure nickel carbonate. The experi- 

 mental method consists in the irradiation of a suspension of the carbonate 

 in pure water, maintained by a stream of carbon dioxide, by the light from 

 an ordinary tungsten filament lamp. The yield of the carbohydrates at 

 various temperatures between 5° and 46° has been determined with con- 

 siderable accuracy. The investigation only became possible when a 

 satisfactory method had been devised for the preparation of pure nickel 

 carbonate. The method consists in the electrolysis of pure water, 

 saturated with carbon dioxide, with nickel electrodes. The carbonate is 

 collected, dried at 100°, and then heated at 140° for thirty minutes. It is 

 then powdered and passed through a 100-mesh sieve, after which it is 



