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BELL SYSTEM TECHNICAL JOURNAL 



in a film of gas on the surface of the crystal, and that this film of gas 

 was thin. The last figures show that this gas film consisted of only a 

 single layer of atoms, and that these atoms were arranged in a crystal- 

 line structure similar to a parallel layer of nickel atoms but separated by 

 distances twice as great as the separations of nickel atoms (Phys. Rev., 

 loc. cit., Fig. 20). This arrangement of gas atoms and surface nickel 

 atoms is shown in Fig. 10. 



Very interesting information was obtained by taking data similar 

 to those represented in Fig. 5 with the crystal maintained slightly 



{100) AZIMUTH 



(110} AZIMUTH 



Sine 



Sin e 



Sin 6 



Fig. 9 — Plots of the grating formulas in the principal azimuths of the crystal. 

 Positions of electron diffraction beams of the fourth type are indicated by crossed 

 circles and positions of beams of the first type by dots. 



warm, at a temperature roughly estimated to be 150° C. For this 

 slightly warm crystal the life history of a typical "Type-3" beam was 

 substantially the same as that shown in Fig. 5. The "Type-4" beams, 

 however, did not develop at all. We concluded that gas atoms settled 

 upon the crystal when warm much the same as when cold, but that 

 upon the warm surface they were unable to take up the regular arrange- 

 ment which they assumed upon the cold surface. One is led to say 

 that the temperature of 150° C. was above the melting point of the two 

 dimensional "gas crystal." It would have been comparatively easy 

 to have found out whether or not the gas crystal melted abruptly at 

 some critical temperature, but this information was not obtained. 

 One of the most striking features of Figs. 6-8 is the marked change 



