SURFACE CHEMISTRY 33 



increased concentration of the adsorbed substance near the surface was 

 thought to be analogous to the retention of the earth's atmosphere by the 

 gravitational attraction of the earth. An adsorbed gas was thus regarded 

 as a kind of miniature atmosphere extending out a short distance from a 

 solid substance. In general such theories were called upon to account only 

 for qualitative aspects of the adsorption of gases on solids. Most of the 

 knowledge of adsorption was empirical. Even Gibbs' law had not been 

 verified experimentally. 



When I first began to work in 1909 in an industrial research laboratory 

 ( I ) (2) , I found that the high vacuum technique which had been developed 

 in incandescent lamp factories, especially after the introduction of the tung- 

 sten filament lamp, was far more advanced than that which had been used in 

 university laboratories. This new technique seemed to open up wonderful 

 opportunities for the study of chemical reactions on surfaces and of the 

 physical properties of surfaces under well defined conditions. I resolved 

 to try the effect of introducing very small quantities of various gases into 

 a highly evacuated bulb containing a tungsten filament. Since by a McLeod 

 gauge pressures as low as lO"^ atmospheres could be measured, it was 

 possible to observe the disappearance of an amount of gas of less than o.i 

 cubic millimeter measured at atmospheric pressure. The use of a tungsten 

 filament presented particular advantages for this could be heated in high 

 vacuum to temperatures of over 3,ooo°K, so that all impurities could readily 

 be distilled out of the filament. The ease and accuracy with which any 

 desired temperature could be produced and measured was also important. 



When a hot body, such as a filament, is in contact with a gas at atmos- 

 pheric pressure in a glass bulb, the convection currents and the uncertain 

 temperature distribution in the gas are factors which greatly complicate 

 any interpretation of observed phenomena involving the interaction of the 

 filament with the gas. 



If the gas, however, is at a pressure as low as 100 baryes,^ the mean 

 free path of the molecules is many times greater than the diameter of 

 the ordinary tungsten filament. Each molecule which strikes the filament 

 has thus made so many collisions with the bulb since its last collision with 

 the filament, that the efifective temperature of the gas in contact with the 

 filament may be taken to be the same as that of the bulb. The disturbing 

 effects of convection currents are then entirely eliminated and the rate of 

 arrival of gas molecules at the surface of the filament can be calculated 

 according to the laws of the kinetic theory of gases. This theory leads to 

 the equation 



'' (2 7rmkT)V2 ' (4) 



* A barye or cgs. unit of pressure, i dyne cm-^, is almost exactly lO"^ atmospheres. 



