Production of Luminosity in Atmospheric Neon. 921 



2. The hypothesis of the orientation of the elementary 

 crystals seems to be the most probable. The appearance of 

 the effect in sputtered plates cannot be argued against this 

 hypothesis, since the sputtered particles are undoubtedly of 

 more than molecular dimensions. 



The fact that no increase of photo-electric current appears 

 until a certain minimum current is reached, indicates that a 

 certain minimum field is necessary to orient an elementary 

 crystal, the further increases being due to the larger numbers 

 so oriented. 



Further, the fact that a second series of readings with a 

 plate gives the irregular results described previously, would 

 seem to indicate that the structure of the metal had been in 

 some way weakened. The greater rises under these con- 

 ditions would likewise support the view. 



The validity of the hypothesis could, however, be best 

 tested by working with plates cut perpendicular to the 

 crystal axis of a large bismuth crystal. A further test 

 might be obtained from the measurement of change of con- 

 tact difference of potential. under the conditions existing in 

 this experiment. 



. Palmer Physical Laboratory, 

 Princeton, N.J. 



LXXXVIII. Critical Electron Velocities for the Production of 

 Luminosity in Atmospheric Neon. By Frank Horton, 

 Sc.D.j Professor of Physics in the University of London, 

 and Ann Catherine Davies, M.Sc, Assistant Lecturer in 

 Physics in the Royal LLolloway College, Englefield Green *. 



IN a recent paper f the authors have shown that in 

 atmospheric neon there are two critical electron 

 velocities associated with the production of radiation from 

 the gas, and three critical electron velocities associated 

 with the production of ionization, the values of these 

 velocities being : 



Minimum radiation velocity . . . ll'S volts. 

 Second radiation velocity . . . 17*8 ,, 

 Minimum ionization velocity . . Kv7 

 Second ionization velocity . . . 20*0 ,. 

 Third ionization velocity .... 22*8 ., 

 In view of the fact that neon is the only gas for which 

 more than one critical ionization velocity has been found 

 under conditions which preclude the possibility oi' the 



* Communicated by the Authors. 



t Proc. Boy. Soc. A. vol. xcviii. p. L24 (1920). 



