SURFACES OF MOLECULES 235 



hydrocarbons increases in geometrical proportion to the length of the chain. 

 It would seem that the principle of independent surface action should 

 afford a means for the development of a simple theory of viscosity which 

 would take fully into account the shapes and sizes of the molecules and of 

 the different chemical groups contained in the molecules. 



The principle has also found many applications in connection with the 

 mechanism of heterogeneous chemical reactions. The interaction of oxygen 

 and hydrogen and of carbon monoxide and oxygen at low pressures in, 

 contact with a heated platinum filament (i8) has shown that equations 

 which are based directly on this principle are in excellent agreement with 

 the experimental results over very wide ranges of pressure and tem- 

 perature. For example, the experiments showed that at low temperatures 

 the reaction velocity was accurately proportional to the partial pressure of 

 oxygen and inversely proportional to the pressure of carbon monoxide. 

 Tills behavior could be completely accounted for by assuming that the rate 

 of reaction was determined by the rate at which oxygen molecules could 

 reach holes in the films of adsorbed carbon monoxide which were left by 

 evaporation of carbon monoxide molecules, or which were formed by the 

 removal of the carbon monoxide by the oxygen which reached these holes. 

 The poisoning effect of the carbon monoxide was due to the fact that the op- 

 portunity for the oxygen to reach the holes was decreased if the carbon 

 monoxide molecules were able to fill up the holes before the oxygen 

 arrived. It seems difficult to reach quantitative agreement with these ex- 

 periments except by the application of the principle of independent surface 

 action. 



The interaction of hydrogen and oxygen in contact with tungsten 

 filaments at temperatures ranging from 1500° to 250o°K is another 

 illustration of a similar kind. Oxygen acts to form WO3 at a rate pro- 

 portional to the oxygen pressure (19). Oxygen atoms in the adsorbed 

 oxygen film do not react with one another and with the tungsten to form 

 WO3 even at the highest temperatures. At i5oo°K the life of oxygen 

 atoms on the surface is of the order of years; at i86o°K it is about 25 

 minutes; and at 2070° K it is 15 seconds. When the atoms leave the films 

 at the higher temperatures they do so as free atoms, not as molecules. 

 Thus, even in the presence of minute pressures of oxygen, the tungsten 

 surface is practically completely covered with a single layer of oxygen 

 atoms. Oxygen molecules which strike this surface condense on it, but 

 evaporate from this second layer at a relatively high rate ; but while thus 

 adsorbed, they move freely over the surface and are able to fill up any 

 holes that form in the first layer. Also the molecules, while adsorbed 

 in the second layer, have a certain probability of interacting with those of 

 the first layer and with the underlying tungsten to form WO3, and the 



