EVAPORATION AND CONDENSATION 



315 



that an atom on the surface will have a sufficient velocity to cause the re- 

 flection of an incident atom. 



The above calculations lead to the conclusion, which is in good accord 

 with experimental facts, that the reflection of tungsten atoms from a tung- 

 sten surface takes place to a negligible degree only. 



By equation (28) we see that the reflectivity of atoms from a surface 

 will depend on the coefficient n and the ratio l/T. It seems probable that n 

 is of the same order of magnitude for all substances and has a value some- 

 where between i and 3. According to Trouton's rule the latent heats of 

 evaporation of different substances are proportional to the absolute boiling 

 points. Nernst finds that this rule is only approximate, and modifies it by 

 placing the ratio l/T proportional to log T. If we consider a series of 

 different substances at "corresponding" temperatures the values of X/T 

 will not differ very greatly, but according to Nernst will be smaller the 

 lower the absolute boiling point of the substance. 



TABLE II 



Table II contains data for a few substances. The absolute boiling point 

 To is given in the second column, while X, the internal latent heat of 

 evaporation at the boiling point, is given in the third. It is seen that the 

 maximum range of the values of X/T is only from 10 to 41, and, since this 

 ratio in (28) occurs under the radical, the value of a2 only varies within 

 the ratio i to 2. The values of 02 and P calculated by taking n = 1 are also 

 given in Table II. 



These data are calculated for the temperature of the boiling point. 

 At lower temperatures a2 would be larger so that P would be still smaller 

 than the values given in the table. 



In the above discussion we have derived a method of calculating the 

 probability that an atom on the surface of a solid may have a sufficient 

 velocity to cause an incident molecule to be reflected. The calculation was 

 based, however, on the assumptions : 



