EVAPORATION AND CONDENSATION 313 



be increased (approximately) by Vi, so that the velocity of A, upon re- 

 turning to the equilibrium position, will be 



The condition that the atom may just be able t o escape from the surface 

 is that this velocity shall be equal to — V2A/M1, in other words, the con- 

 dition is ^ 



[2X 



./= -(i+/3)^^- (24) 



Now let Ui and ao be the ratios between the initial velocities of A and 

 C respectively, and the mean velocities due to thermal agitation. Thus, 



«, = ^^ and a2 = y^. (25) 



where the mean velocities of thermal agitation V\ and V2 are given by 



'^ = J^ ^"' '^ = j 





Substitute (25) and (26) in (22), and the resulting value of Vi' in 

 (24). Then substitute the value of /5 from (20), and remembering that 

 M1/M2 = Y we obtain our final equation : 



\a\{y 



UT-, in I 2X 



This equation furnishes us an approximate method of estimating the 

 effect of various factors upon the reflection of atoms from a surface. This 

 will best be illustrated by a few examples. 



First Case: nii = m2. 



In the first place, let us consider the case of a substance in equilibrium 

 with its own saturated vapor. We then have Ti = T2 and Wi = W2, so 

 that Y — I • 



If we express l and R in calories (i? = 2). then equation (27) re- 

 duces to 



n IT 



This equation tells us how large a^ must be in order that the atoms of 



