SURFACES OF DISCONTINUITY 567 



tension did not differ appreciably from that of water, but as 

 the area was reduced below this value, the surface tension 

 fell rapidly. Later, Rayleigh (Phil Mag., 48, 321, (1899)) 

 suggested that at this critical area the molecules are just 

 crowded into a layer one molecule thick; that they are in fact 

 floating objects which begin to repel one another when coming 

 into contact in a single layer. This accounts for the first 

 appearance of a diminution in surface tension at this point; a 

 barrier moving a small distance in the direction of the pressure 

 arising from this would gain kinetic energy, presently dissipated 

 in the general body of the fluid. The corresponding loss of 

 energy will be found in the fact that the expanding surface 

 covered by oil will not gain as much surface energy as is lost 

 at the contracting clean surface, which is merely a statement of 

 the fact that the oil covered surface has a smaller "surface 

 tension" than the clean, but does not imply the existence of a 

 physical tangential pull in the surface. Actually, as Devaux 

 was the first to point out, some films may acquire the properties 

 of a two-dimensional solid possessing a tangential rigidity in 

 the surface which prevents them being blown about into differ- 

 ing shapes. 



£5. The Work of Langmuir and Adam. The Concept of ^'Surface 

 Pressure." Equations of Condition for Surface Phases 



Great improvements in the experimental appliances were 

 introduced by Langmuir (J. Am. Chem. Soc, 39, 1848, (1917)) 

 so that it became possible to measure these small surface pres- 

 sures, and his work has been extended with great success by 

 Adam. In Adam's book, already cited, will be found an 

 account of his work with references to the numerous papers by 

 himself and his co-workers. In the most recent form of Adam's 

 apparatus surface pressures as small as 0.01 dyne per cm. can 

 be measured. Also a great many tests have been made with 

 substances which are simpler than oils and whose chemical 

 constitution is better known. It is possible actually to give the 

 results in terms of the surface pressure corresponding to the 

 area of surface covered by a known number of molecules. 

 Thus, for the normal saturated fatty acids, no trace of surface 



