596 RICE ART. L 



Thus "the heat of adsorption is independent of the particular 

 value of n, so that each equal increment in the amount adsorbed 

 is accompanied by the same heat evolution. This is, of course, 

 only possible when the adsorbed layer is so diffuse that the 

 amount already adsorbed has no effect on further adsorption." 

 Micheli calculates the heats of adsorption for pentane, hexane, 

 and heptane, and finds them to be 13.6, 11.7, 14.6. He notes 

 that the probable error may amount to 20 per cent and so he 

 takes the three results to be roughly the same ; at all events they 

 do not show any sign of increasing with the number of carbon 

 atoms in the molecule; even an accuracy of only 20 per cent 

 precludes that possibility. From this he concludes that the 

 molecules do not lie flat on the surface, for then we should 

 expect the heat of adsorption to be roughly proportional to the 

 number of carbon atoms in the molecule. (The reader will 

 recall a similar line of argument by Langmuir in section VII 

 of this commentary.) * 'These considerations, then, furnish some 

 additional support for the conclusion that an end CH3 group 

 forms the only point of attachment to the water surface." 



A good deal of work on heat of adsorption and "heat of 

 wetting" has been carried out at the interfaces between solids 

 and gases or vapors, but reference to this will be deferred until 

 we reach the subsection of Gibbs' treatise which deals with 

 solid-fluid interfaces. 



S6. Dependence of a on the ''Age" of a Surface 



With reference to the subject discussed on pp. 272-274 of 

 Gibbs' work, namely, the effect on the surface tension of creat- 

 ing a fresh surface, it may not be out of place to mention the 

 suggestion sometimes made, that because ordinary liquids, even 

 "pure," are constituted really of different molecules (since they 

 differ in degree of polymerization or chemical activity) they 

 should display a surface tension different at a fresh surface from 

 that which would exist there some time after formation. This 

 argument is clearly based on the adsorption law and the assump- 

 tion that there are at least two types of molecules in the liquid, 

 one of which produces a higher surface tension than the other. 

 On forming a fresh surface, the composition of the surface layer 



