﻿690 Mr. D. C. Henry on a 



Suppose there are N such points o£ attachment. If a 

 molecule from the gaseous phase impinges on a point of 

 attachment unoccupied by any other adsorbed molecule, it 

 will condense, forming a single "adsorbed layer"; if it 

 impinges on a point already occupied, it will also condense, 

 forming a second layer. But the relative life of a molecule 

 on the surface will depend on the attractive force exerted on 

 it by the surface, and if, as is usually the case, the attractive 

 force between molecules of adsorbent and adsorbate is much 

 greater than that between two molecules of adsorbate, the 

 relative life of a molecule in the second layer will be so small 

 compared with that of a molecule in the first layer, that we 

 may treat molecules impinging on points already occupied 

 as if they were immediately reflected and never condensed. 



At any moment let a fraction O of the points of attach- 

 ment be vacant, and fractions #,, 2i ... 6 n be covered with 

 monomolecular layers of S l5 S 2 , ... S w respectively ; then 



#0+01 + 02+ ...+0n=l (1) 



Let a single molecule of Si, S S ,-...S» occupy respectively 

 a\, a 2 , ... a n points of attachment * ; the number of molecules 



of component S r adsorbed will be — — -, and the adsorbed 



Clip 



quantity in gram-molecules will be 



x -S> • n 



where N is Avogadro's constant (6*06 X 10 23 ). 



Now the rate of evaporation from the surface will be 

 determined by Maxwell's distribution law as the number of 

 molecules which reach, per second, a state of agitation 

 sufficient to break free from the force field, and is given, 

 for S r , in gram-molecules per second, by the expression f 



VVl.a ET .X r =(say)^X r , . . . (3) 

 where \ r is the internal heat of evaporation of S,. from the 

 surface, A r is a constant depending on the field of force, 

 and R and T have their usual meanings ; v v is written 

 for the expression 



_ V 



A r s/T.e r t\ 



* Since the number of points of attachment occupied by a molecule is 

 not subject to a merely geometrical restriction, but is determined by the 

 field of force, it does not appear essential that the quantities a should 

 be integers. r 



t Langmuir, Jour. Amer. Chem. Soc. xxxv. p. 122 (1913). Richardson, 

 Phil. Trans. A. cci. p. 501 (1903). 



