23 6 PHENOMENA, ATOMS, AND MOLECULES 



holes thus formed by the removal of oxygen from the first layer are soon 

 filled up by the migration of the molecules of the second layer. 



Hydrogen molecules are not able at any temperature to' react directly 

 with the oxygen in either of the first or second layers (lo), but if they 

 reach the tungsten at one of the holes in the first layer, they react im- 

 mediately with the adjacent oxygen. Thus when the oxygen pressure falls 

 to a certain low critical value, the hydrogen completely removes all the 

 .oxygen from the surface, and it does this suddenly. The hydrogen which 

 then strikes the film is dissociated into atoms to an extent that depends 

 upon the temperature, and the atomic hydrogen formed goes to the bulb 

 and reacts with the WO3 which has previously been deposited there. 



This theory which can readily be stated quantitatively appears to be in 

 complete agreement with all the experimental facts, so that again we find 

 support for the principle of independent surface action. 



The molecules in adsorbed films on solutions of organic substances in 

 water are frequently in a state of a two-dimensional gas. Oil films on water 

 may exist as solid or liquid films. In the case of so-called expanded 

 films (12), the heads of the molecules act as a two-dimensional gas, while 

 the tails form a two-dimensional liquid. The molecules of oils adsorbed on 

 solids, such as those that are responsible for some of the lubricating 

 properties of oils, and those involved in the phenomena of flotation of ores, 

 usually show little or no tendency to move over the surface. In other words, 

 the molecules appear to be attached rigidly to the surface. 



Many of the equations that were developed in the quantitative studies 

 of the velocities of heterogeneous reactions assumed that bare spots on the 

 surfaces were distributed over the surface according to statistical laws. 

 Such statistical distribution would not occur unless the molecules possessed 

 a certain degree of mobility over the surface, for the molecules that are 

 removed by the reaction are those that are adjacent to holes already exist- 

 ing. Volmer and others have shown experimentally that adsorbed atoms 

 frequently possess great mobility even on solid surfaces and thus act like 

 two-dimensional gases of high viscosity. 



I believe that the principle of independent surface action will be useful 

 in studying many properties of organic substances which have hitherto been 

 too complicated to be treated quantitatively. W^ithin recent years I have 

 made some attempts of this kind. 



When a liquid is separated into two parts, along a surface having an area 

 of I sq. cm., two new surfaces with a total area of 2 sq. cm. are formed. 

 The surface tension or free surface energy measures the work done per 

 unit area in forming the new surfaces. The total surface energy y, which is 

 equal to the free surface energy extrapolated back to the absolute zero, 



