ox ABSORPTION 8PECTKA OF ORGANIC COMPOUNDS. 311 



restricted to tliose reactant molecules wliicli are in actual coiiUct witii its 

 surface. The activated layer of reactant molecules can therefore be only one 

 molecule deep, and this has been experimentally established. 



In the case of highly endothermic r(!actions in which the amount of energy, E, 

 required to activate a single molecule is large — that is to say, a large number of 

 molecular quanta — considerable difhculty will be found in realising them when 

 tiie energy is supplied by either the first or the second method. They can, 

 however, be induced when the energy is supplied at the phase frequency, for 

 one phase quantum is sufficient to change a molecule into any phase. Such 

 reactions are grouped under the general term 'photochemical, since the phase 

 frequency always lies in the visible or ultra-violet region of the spectrum. 



A very important deduction may be made from the existence of the first stage 

 of a reaction — namely, that for every increment of energy, E, absorbed one mole- 

 cule must undergo reaction. Since in all photochemical reactions the amount of 

 energy absorbed per molecule is one phase quantum, the deduction in its simplest 

 form states that in all photochemical reactions one molecule must react for every 

 quantum of light energy absorbed. This is known as Einstein's law of photo- 

 chemical equivalence, and was enunciated by him on the basis of Planck's energy 

 quantum theory. This law can very easily be put to the test of experiment, but 

 it has been found that in every case, except the photochemical ozonisation of 

 oxygen, the number of reacting molecules is very much greater than one for 

 every quantum of energy absorbed. In some reactions as many as 10,000,000 

 molecules react for every quantum absorbed. Not only has this extraordinary 

 divergence given rise to serious criticism of the energy quantum theory, but it 

 has even led to the view that there is no connection between radiant energy and 

 chemical reaction. 



The molecular phase theory gives a complete explanation of this divergence 

 from Einstein's law — an explanation which has been experimentally proved, and 

 which has considerably advanced our knowledge of photosynthetic processes in 

 general. Indeed, the theory receives its greatest support from this phenomenon. 

 The application of the theory to the problem is very simple, and merely takes 

 into account the energy that is evolved in the second and third stages of the 

 reaction. An exothermic photochemical reaction may be considered of a sub- 

 stance with phase quantum equal to 34 molecular quanta, and for which the 

 necessary increment of energy, E, per molecule is 2 molecular quanta. On 

 exposing this substance to light of frec^uency equal to its phase frequency, the 

 molecules will each absorb one phase quantum of 34 molecular quanta and become 

 activated. Let a single molecule absorb one such quantum and be converted 

 into the active phase. For this phase change two molecular quanta are required, 

 and therefore 32 molecular quanta are evolved. Since this energy is radiated at 

 the characteristic infra-red frequencies of the molecule it can be re-absorbed 

 by the surrounding molecules, so that under optimum conditions the absorption 

 of one single phase quantum may cause the activation of 17 molecules. This 

 is the first reason for the divergence from Einstein's law. 



The instant that a molecule is activated it reacts, and the second and third 

 stages of the reaction ensue with the evolution of the energy F + G. This 

 energy is radiated as heat^ — that is to say, although it is equal in amount to 

 a whole number of molecular quanta characteristic of the resultant molecules, it 

 is radiated at the atomic and infra-molecular frequencies characteristic of these 

 molecules. Whatever the actual reaction may be, the reactant and resultant 

 molecules must possess some atoms in common, and therefore frequencies in 

 common. The energy radiated during the second and third stages of the re- 

 action of a molecule must be wholly or partly reabsorbed by the surrounding 

 molecules, with the result that these become activated and react. This is the 

 second reason for the divergence from Einstein's law. It may readily be under- 

 stood that when E is small and F + G is large the divergence will become enor- 

 mously great if the proportion of the radiated energy that is reabsorbed is large. 

 This proportion will depend on two factors, the density of the reacting sub- 

 stance and the density of the radiated energy; and if the phase theory explanation 

 is correct, the divergence from Einstein's law will rapidly increase when either 

 the 'density of the absorbing substance or the density of the radiated energy is 

 increas&d. There is abundant evidence that the divergence from Einstein's law 



