318 PHENOMENA, ATOMS, AND MOLECULES 



The last column of the table gives the ratio of the time of relaxation to 

 the natural period of oscillation of the atoms. In the case of the metallic 

 substances this ratio is very much less than unity, which must mean that 

 thermal equilibrium is established between adjacent atoms in a small frac- 

 tion of the time necessary to complete a single oscillation. 



Under these conditions it is apparent that our second assumption 

 (elasticity in collisions) is not justified in the case of metals. It is probable, 

 in fact, that atoms of metal vapor striking the surface reach practically 

 perfect thermal equilibrium with adjacent atoms before they can possibly 

 escape again from the surface. The amount of reflection during the con- 

 densation of metallic vapors is therefore probably extremely small. Since 

 the high heat conductivity of metals is related to their electrical conduc- 

 tivity, we may conclude that "free electrons" play an important part in the 

 mechanism of the condensation of metallic vapors. 



From Table III we see that even in the substances of low heat con- 

 ductivity such as ice or sulfur, the "rate of relaxation" is so high that the 

 collisions must be far from elastic. A slight loss of energy to adjacent 

 atoms, must have a very great effect in decreasing the chance that an 

 incident atom or molecule will be able to escape, so that even with non- 

 metallic substances it is probable that the reflection of molecules occurs to 

 a much smaller degree than would be estimated from the values of P 

 given in Table II. 



However, it would not be safe to conclude that in all cases where 

 molecules strike similar molecules (or atoms) on a surface that the re- 

 flectivity must be negligibly small. We have seen that there is evidence 

 that metals in contact with hydrogen even at low pressures have their 

 surface completely covered with a layer of hydrogen atoms or molecules. 

 The accommodation coefficient 0.25 observed for hydrogen in contact with 

 platinum, indicates that a considerable fraction of the hydrogen molecules 

 striking other hydrogen atoms or molecules on the surface is reflected. The 

 reasons for the high reflectivity in this case probably are : 



I. The heat of condensation X is very small. By Table II we see that 

 ?L for liquid hydrogen at its boiling point is only 205 calories per gram 

 molecule. It is probable, however, that the hydrogen atoms on a metal •" 

 surface are held by very strong chemical forces ^* so that they are much 

 more thoroughly saturated than the atoms or molecules in liquid hydrogen. 

 There is good reason to think, therefore, that such a layer of hydrogen 

 atoms would exert only very weak attractive forces on incident hydrogen 

 molecules, and that the value of 1 for the condensation of the second layer 

 would be much less than 205 calories. 



^^ There is reason to think that for the condensation of the first layer of atoms 

 of hydrogen on platinum the value of "k is about 40,000 calories per gram molecule. 



