580 RICE AET. L 



where C and A are given at the top of page 268. Now 7/ is 

 the density of the Hquid and 71" is the density of the Hquid's 

 vapor in the gaseous phase, so that 71" is very much smaller 

 than 7/; 72" is the density of the gas or vapor, whose adsorption 

 is being considered, in the gaseous phase ; 72' its density in the 

 liquid bulk phase, may be regarded as zero. Hence, practically, 



A = -7iV, 



C = ri72" + r2(7/ - 7/0 

 = ri72" + r27i' . 



Therefore 



c _ _ r3_ _ £2^ 



A ~ 7/ 72" ' 



Since Ti is zero by the choice of dividing surface, it follows that 



C _ _ £2 

 A " ~ 72" 



or 



da 

 dp 



where 7 refers to the density of the adsorbed vapor in the 

 gaseous phase.* Before passing on to consider the experi- 

 mental results we may remind the reader of the mechanical 

 explanation of gaseous adsorption given m the last paragraph 

 of section IV of this article. The existence of a surface energy 

 depends, as we saw, on a normal field of force existing in a 

 molecular layer at the surface of the liquid and also extending a 

 similar distance into the space above the liquid. Such a field 

 would cause an increased concentration of gas close to the sur- 

 face, just as the density of the atmosphere is greatest at the 

 lowest level in the earth's gravitational field. Actually the 

 outward attraction of this concentrated layer of gas would 



* Not of the liquid's vapor; 7/' is the density of that. 



