Mr Rideal, On the latent heats of vaporisation 295 



its vapour the number of molecules striking the surface in unit 

 time is equal to the number leaving. 



According to the effusion equation developed by Herz, Knudsen 

 and Langmuir* the number of molecules striking unit area per 

 second in a gas at pressure p, temperature T and of molecular 

 weight M is given by the equation 



P 



^ ~ V27tRMT' 

 where fx is the number of gm. mols. striking a square cm. per 

 second. In a recent communicationf it was shown that the rate of 

 a unimolecular reaction 



dn -^^^ 



had a physical significance in that v was the time of molecular 

 relaxation. This relationship shown to be experimentally true by 

 Dushman and Langmuir J assumes merely that intermolecular 

 energv changes involving chemical forces obey the quantum re- 

 lationship. The above data indicate that the latent energy of 

 evaporation is the work done in overcoming forces of a nature 

 similar if not identical with the usual chemical manifestations. 

 We may therefore regard the vaporisation of a substance from a 

 hquid or soUd surface as a monomolecular chemical reaction. 

 The number of gm. molecules per square cm. of surface is 



where N is the number of molecules per gm. mol. and d the molecu- 



. The rate of evaporation in gm. mols. per square cm. per sec. is 

 accordingly given by the expression 



^ Nhv 



e"BT . 



NTTd^ 



This can be equated to the number of gm. mols. striking a square 

 cm. per second, or 



P V _Nhv 



, = e RT . 



V27TRMT ^"-^d^ 



On taking logs of each side we obtain 



Nhv , V V2ttMR 



log. P-- gT+i 1°8 ^ + '»«' -Nd^ ■ 



* Ph>is. Rer. -2, 331 (1913). 



t Rideal, Phil. Mag. xi, 461 (1920). 



i Jour. Amer. Chem. Soc. 42, 2190 (1920). 



