156 Prof. E. K. Rideal on the Critical 



Similarly, i£ v r is the displacement o£ the centroid of 

 those molecules whose equilibrium positions form the " net" 

 in the plane z — r, 



and these centroids oscillate in the manner described in § 3. 



Bedford College, 



Regent's Park, N.W. 1. 

 February 1921. 



XIV. On the Critical Energy Increment and - Trout on s Rule. 

 By Eric K. Rideal *. 



IN a molecular assemblage, the distribution of the 

 molecules according to their kinetic energies can 

 be calculated by means of Maxwell's distribution law. 

 The phenomena of physical and chemical action indicate 

 that the molecules differ from one another not only in 

 kinetic energy but also in activity. The activity of a 

 molecule appears to be related to the position of a valence 

 electron relative to the nucleus, since it can be calculated 

 by means of the quantum relationship and is frequently 

 associated with an absorption or spectral line in the 

 spectrum of the substance. 



The calculation of the chemical critical energy increment 

 for a molecular species from radiation or temperature 

 coefficient data leads to a value which is not unique for 

 the particular reactant. In the first case, if a substance A 

 undergoes reaction with two activated substances B and C, 

 the critical energy increments of A for the two reactions 

 are not necessarily the same ; e. g., the critical energy 

 increment of chlorine for combination with sodium is 

 many times greater than the value necessary for the 

 reaction of evaporation. Occasionally also the combination 

 between two substances A and B can be effected in dif- 

 ferent ways entailing different critical energy increments; 

 amongst such cases may be mentioned the decomposition 

 of HI (Lewis, J. C. S. cxiii. p. 489, 1918) and the 

 formation of ozone (Rideal and Kunz, Journ. Phys. Chem. 

 v. p. 379, 1920). The two conditions to be fulfilled are 

 firstly (Perrin, Ann. de Physique, xi. p. 51, 1920) that the 

 critical energy increments shall be determinable from 

 the quantum relationship, and secondly that the law of 

 » Communicated by the Author. 



