B.— CHEmSTRY. 53 



is that of F. F. Blackman, who postulates a second reaction due to an 

 enzyme, superimposed on the first. In the laboratory experiments the 

 Blackman reaction must obviously be absent, and in spite of this the 

 results are remarkably analogous to those found in the living plant. 

 The analogy is made still closer by the fact that the linear relation shown 

 in fig. 3 gives the temperature coefficient of the laboratory photosynthesis 

 between 20° and 30° as 1-54, whereas the value found in the plant is 1-6. 



This, however, is by the way, for the analogy that is particularly 

 striking is that between photosynthesis and photoluminescence, both of 

 which have been found to have an upper and a lower temperature limit. 



The success that has attended the application of the hypothesis of 

 complex formation to three widely differing phenomena justify its 

 general application to all thermal chemical reactions. This naturally 

 leads to the view that every such reaction depends on the presence of a 

 catalyst. There seems little objection to this because it is a fact familiar 

 to everyone that chemical reactivity suffers a most remarkable decrease 

 as all impurities are removed. It is perhaps a sweeping statement to 

 make that no thermal reaction can take place in the complete absence of 

 a catalyst, but the fact remains that in every case which has been accurately 

 examined the reaction velocity is zero. In inorganic chemistry the most 

 effective catalyst is water and H. B. Baker's work on the absence of 

 reaction between dry substances is classical. It may be that this power 

 of water is connected with its great ionising power towards inorganic 

 salts, for it is possible that ionisation itself is the result of a complex 

 between solvent and solute. 



In general, it must be remembered that every chemical reaction has 

 its own critical increment of energy, and this means that the reactant 

 molecules must be raised to a definite energy level which is specific for 

 the reaction required. The catalyst molecule must, therefore, be one 

 which by forming a complex with the reactant molecule raises it to that 

 energy level and no other. The possibility of the same molecules being 

 raised to different energy levels has been established by absorption 

 spectra, since by the use of different solvents it is possible in the case of 

 many compounds to obtain them in different physical states a'^ evidenced 

 by different absorption bands. The integral relation has been suggested 

 in photochemical reaction, namely 



where 7(v„ is the quantum absorbed at the visible f»r ultra-violet frequency 

 characteristic of the reactant molecules in their initial state, /.Vj is the 

 critical quantum of activation, and /.Vois a quantum of fluorescence and 

 also the quantum absorbed at the characteristic frequency of the activated 

 molecule. If this relation be fully confirmed by further work, the 

 different molecular states of one compound, proved by absorption spectra 

 methods to exist in different solvents, will be directly linked up with the 

 different chemically reactive states of that compound. The difficulty in 

 postulating a series of catalysts which can induce different reactions of 

 the same substance will then disappear. 



We may now turn once again to the radiation hypothesis and take 

 stock of the position. The protagonists of this theory, after enunciating 



