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LI. On the Velocity of Unimolecular Reactions. By Eric 

 K. Rideal, Professor of Physical Chemistry at the Uni- 

 versity of Illinois, U.S.A.* 



IN a recent communication (Phil. Mag. vol. xxxix. p. 2(j, 

 1920) \V. C. M. Lewis has drawn attention to the 

 anomalous results obtained in the calculation of the velocity 

 constant of a unimolecular chemical reaction, the decom- 

 position of phosphine, from the standpoint of the radiation 

 hypothesis. 



This lack of agreement in a monomolecular reaction is 

 all the more serious, since if the correct solution could be 

 found the radiation hypothesis could be extended to the 

 vaporization of metals, thus permitting us to calculate 

 the so-called " chemical constants " of substances with the 

 aid of the Clausius-Olapeyron and Knudsen relationships, in 

 terms of the molecular diameters and the natural radiation 

 frequencies. 



Drs. Dushman and Langmuir have recently indicated that 

 Trautz's values for x in the following general equation, 



dn -3 



dt - " RT ' 



are proportional to Q. 



This purely empyric relationship has, however, a theo- 

 retical basis, and in the light of the radiation theory leads to 

 remarkable conclusions. 



In the general equation for a monomolecular reaction, 

 Q is replaced by the quantum relationship Q = N/w. 



Hence the rate of change per molecule per unit of time is 



dn yr -*? 



Tt = Ke "'• 



In this expression K has the dimensions of t~ l and is the 

 reciprocal of the time necessary for the molecule to pass from 

 the inactive to the active condition, and is thus identical with 

 Perrin's molecular " sensibilite " (Ann.de Phys. ii. p. 21, 

 1919) and with the inverse of Maxwell's " time of molecular 

 relaxation. " 



An approximation to the values of K or the inverse of the 

 times of molecular relaxation may be obtained from thermal 

 conductivity data. A few of these are appended in the 

 following table, the calculations being performed according 



* Communicated bv the Author. 



