B.— CFIEMISTRY. 55 



A second method of activation has been suggested, namely the forma- 

 tion of a complex between a molecule of the reactant and a molecule of 

 a catalyst, in which the former has gained its critical quantum of activation 

 at the expense of the rotational energy of the latter. Such a complex 

 will be stable and will only be resolved into its components when the 

 defect in rotational energy of the catalyst molecule has been restored, 

 this being possible by the absorption of infra-red radiation. The result 

 of this resolution will be the setting free of the reactant molecule in the 

 activated state. It follows that the complex will only be stable below a 

 certain definite temperature. As the temperature is progressively raised 

 the stability will be progressively decreased, until a second temperature 

 limit is reached, at which the complex has no stability. At this upper 

 temperature the reaction velocity will be a maximum. The observations 

 of absorption spectra afford strong support to this hypothesis of complex 

 formation. The particular case of the phenolic ethers has been examined 

 in detail, and it has been found that in concentrated sulphuric acid solu- 

 tions a stable state exists at 15°, in which the ether molecules have received 

 their critical quanta of activation. A progressive increase of temperature 

 causes a progressive increase in reaction velocity. 



In applying this hypothesis to all thermal reactions it is necessary to 

 assume first that no reaction can occur in the absence of a catalyst. This 

 assumption seems to be justified by the known effect of the removal of 

 all impurities on the reaction velocity. In the second place it is necessary 

 that the catalj^st activate the reactant molecule to the energy level required 

 and no other. That this is possible is established by absorption spectra 

 observations, which show that the same molecules can be raised to different 

 energy levels within the complexes formed with different solvents. In 

 inorganic chemistry the problem is less complicated, since in the great 

 majority of cases only one activated state is indicated ; this activated 

 state exists in general within the complexes formed with water. 



In the field of photoluminescence the activation is a two-stage process, 

 since phosphorogen molecules are already partially activated in their 

 complexes with the molecules of the diluent. The existence of these 

 complexes is proved by the absorption frequencies of the phosphorogen, 

 which are nearer the longer wave-lengths than those of the same substance 

 in the free state. By photo-activation the phosphorogen molecule within 

 the complex is raised to a higher energy level, the process being attended 

 by the radiation of a quantum of fluorescence. This higher energy state 

 is stable since the complex still exists. It follows that there will be a 

 temperature limit below which no phosphorescence can take place. As 

 the temperature is progressively raised above this limit the intensity of 

 the phosphorescence will progressively increase and the persistence will 

 progressively decrease. When, by further increase in temperature, the 

 region of complete instability is entered, the conditions for the special 

 photo-activation no longer exist and all luminescence ceases. Not only 

 are the two temperature limits of photoluminescence explained by the 

 hypothesis of complex formation, but also the stability of the activated 

 states. 



The reaction whereby carbohydrates are photosynthesised from 

 carbonic acid may be compared with the photo-activation of a phosphore, 



