EVAPORATION AND CONDENSATION 319 



We see by equation (28) that a small value of I. would tend to increase 

 the number of atoms reflected. 



2. The temperature is very high compared to the boiling point of 

 hydrogen, and this fact, according to (28) and (30) , would tend to increase 

 the chance of reflection. It may also decrease X. Although the heat of 

 evaporation of liquids decreases to zero when the critical temperature is 

 reached, we must not consider that X also disappears at this temperature. 

 In the sense in which we are using this quantity it represents the work 

 done in separating the atoms of the solid or liquid to an infinite distance 

 from each other. This quantity evidently may have considerable values even 

 at temperatures much above the critical temperature. 



3. The atoms forming the adsorbed layer on the surface may be so 

 rigidly held by the underlying metal as to greatly increase the tendency 

 for the weakly attracted incident atoms to be reflected. In deriving equa- 

 tion 28 it was assumed that during the time of a collision, the atom on the 

 surface is free to move according to the usual laws of elastic collisions. 

 If the adsorbed atoms are rigidly held to the metal atoms the effective mass 

 of the adsorbed atoms would be greatly increased. According to equation 

 (27), this would tend to increase the reflectivity. 



Another effect produced by the close coupling of the adsorbed atoms to 

 the metal and the loose coupling to the incident atoms is to lengthen the 

 time of relaxation of the incident atoms. 



Second Case. Unlike Atoms. — So far, we have considered the condensa- 

 tion of a vapor on a solid whose surface consists of the same kind of atoms. 

 If atoms of a light gas strike the surface of a solid consisting of heavy 

 molecules, the case becomes more complicated, since we must use equation 

 ( 2y) instead of (28). Let us consider the case of hydrogen molecules strik- 

 ing an absolutely clean surface of tungsten or other heavy metal. For such 

 a case we may put approximately y --= .01, so that, if we place 7\ = T^ = T 

 and i? = 2, equation (27) becomes 



From this equation we may estimate the amount of reflection which 

 would occur if the directions of all motions and forces were normal to the 

 surface. Under actual conditions the amount of reflection would be con- 

 siderably smaller. In making this calculation we substitute in (35) the 

 value of n, y, and T and then choose particular values of ai, and find ao 

 by the equation. Thus if we place 11 = i and y = 40,000, we find the pairs 

 of values of a^ and as given in the first two columns of Table IV. 



The value of 02 gives the maximum (positive) velocity which an atom 

 of the solid may have and still cause the reflection of an atom colliding 



«2 - 4-95 ai = - 0.058 



