582 RICE AKT. L 



There appears, however, to be a somewhat abrupt change from a 

 simple adsorption process to a condensation." In later work Ire- 

 dale examined more carefully the remarkable behavior exhibited 

 at the saturation point of the vapor. Among the vapors studied 

 was water vapor in the presence of air. In this case the slope 

 of the {a, p) curve was practically uniform up to the saturation 

 point, and so the adsorption increased uniformly with the den- 

 sity and partial pressure of the vapor right up to the satu- 

 ration point. Calculation of r at this point gives a value 

 1.8 X 10~^ gram per sq. cm. which is somewhat less than that 

 required for a unimolecular film (3.8 X 10~^ gram per sq. cm. 

 according to Langmuir). At the saturation point there is the 

 same instability in the tension of the vapor-mercury interface, 

 its value being entirely uncontrollable and lying anywhere 

 between 447 and 368 dynes per cm. Iredale suggests that 

 the primary phenomenon is the gradual formation of a uni- 

 molecular layer, this being represented by the earlier portion of 

 the curve. After the vapor reaches the saturation value a 

 very thin film of liquid may be produced, the thickness of which 

 "is not a determinate function of the pressure and temperature, 

 though the most stable state corresponds to the formation of a 

 film, which may, from the standpoint of intermolecular forces, 

 be regarded as infinitely thick." Iredale also examined the 

 adsorption of benzene vapor on a mercury surface. This 

 showed one rather unexpected feature. He considered that 

 near the saturation point the value of r attained a maximum 

 and decreased slightly with a further small increase of pressure. 

 He also found a similar tendency in methyl acetate, though 

 not in water vapor. (This was criticized later by Micheli 

 whose work we shall refer to presently.) The maximum value 

 for benzene was such as agreed with an area 21 X 10~^^ sq. 

 cm. for each molecule, very near to Adam's value (23.8 X 10~^^) 

 for certain benzene derivatives on a water surface, and once 

 more supported the view that the vapors adsorbed on the 

 surface of mercury tend to form primary unimolecular films. 

 Further measurements were made using the sessile drop method 

 for measuring surface tension, and without admixture of air. 

 These results were in fair agreement with the previous work and 



