APPLICATION OF ELECTRON DIFFRACTION 603 



They are to be compared directly with Figs. 6-8. Better agreement 

 could not be expected in view of the fewness of the experimental data. 

 For other assumed separations the calculated wave-lengths of the 

 intensity maxima do not agree so well with the observations. 



Designating azimuths as shown in Fig. 10 is tantamount to stating 

 that the gas atoms locate themselves directly over nickel atoms of the 

 third layer. This location appeals to one as inherently probable. The 

 separation of 3.0 A.^^ is greater by about fifty per cent than the separa- 

 tion of layers of nickel atoms parallel to the surface (2.03 A.). This 

 also seems reasonable in view of the fact that the gas atoms are 

 separated laterally by distances twice as great as the separations of 

 nickel atoms. 



Our analyses of the diffraction patterns of the first, third and fourth 

 types seem to us to be fairly satisfactory. The study of the diffraction 

 pattern of the second type has, however, turned out to be futile. This 

 pattern consisted of twelve diffraction beams, one in each of the 

 principal azimuths of the crystal (see Fig. 10). All these beams ap- 

 peared at the same co-latitude angle of about 58° and for the electron 

 wave-length 1.17 A. We know that this curious pattern had its origin 

 in a heavy layer of adsorbed gas, but we have been unable to determine 

 what were the crystalline regularities in this layer. Our present feeling 

 is that these beams may really have reached their intensity maxima at 

 slightly different angles and wave-lengths in the different crystal 

 azimuths, and that perhaps there may have been other beams belonging 

 to this pattern which we did not detect. We are inclined to believe 

 that we shall not be able to obtain information concerning the crystal- 

 line arrangement of a heavy gas layer until further experimental data 

 are available. 



Looking back over the information recorded here concerning gas 

 upon a crystal surface, one should think of it as information which has 

 been obtained by a new method of diffraction analysis. Thinking of it 

 in this way I realize that the observations are of a rather elementary 

 nature. The facts which we have discovered are by no means com- 

 parable in complexity with the elaborately detailed information con- 

 cerning crystal structure which is so readily obtained by X-ray analy- 

 sis. 



Electron diffraction is, however, a new field. At the time when the 



" Logically this value of 3.0 A. should perhaps be changed by a few per cent to 

 take into account a refractive index slightly different from unity. 



