.922 Prof, florton and Miss Davies on Electron Velocities 



higher critical velocities being concerned with the ejection 

 of a second electron from an already ionized atom, it 

 seems desirable to mention briefly the precautions taken to 

 ensure the purity of the gas used in these experiments. 

 The neon was purified by Dr. Aston of Cambridge, using 

 the elaborate fractionation apparatus which he devised 

 for this purpose. The process of purification was accom- 

 panied by frequent determinations of the density of the 

 gas, and was continued long beyond the stage when no 

 alteration of density could be detected. In admitting the 

 gas to the apparatus adequate precautions were taken to 

 remove any impurities which might have found their way 

 into the neon from the glass walls of the vessel in which 

 it was stored, or from the walls of the tubes through 

 which it passed, and, during most of the experiments, the 

 pure gas was slowly streaming through the ionization- 

 chamber. With these precautions the only possible im- 

 purity in the gas used was helium, and in view of the 

 lengthy and careful treatment of Dr. Aston it is unlikely 

 that more than a minute trace of helium was present. 

 A spectroscopic examination of the gas showed no lines 

 but those of the neon spectrum. The presence of helium 

 in sufficient quantity might have accounted for the detection 

 of a critical ionization velocity at 20 volts — approximately 

 the resonance velocity for electrons in helium, — but the 

 experiments about to be described show that this electron 

 velocity is essential for the production of the principal series 

 imps in the neon spectrum. 



In view of the results of the positive ray experiments 

 of Sir J. J. Thomson * and of the more recent experiments 

 of Dr. Aston |, which have shown that atmospheric neon 

 contains two constituents of atomic weights 20'00 and 22-00 

 and possibly a small proportion of a third constituent of 

 atomic weight 21, the detection of three critical electron 

 velocities for ionization of the gas is of particular interest. 

 In order to explain his results, Dr. Aston assumed that the 

 constituents of atmospheric neon are isotopes, an assumption 

 which is justified by the fact that according to Moseley's 

 theory of atomic numbers there can be no unknown element 

 of atomic weight between 20 and 23. The existence of 

 three isotopes would not, however, be expected to lead 

 to three different ionization velocities, since the atoms of 

 isotopes have the same nuclear charge and the same number 

 of surrounding electrons, which would presumably be dis- 



* J. J. Thomson. ' Rays of Positive Electricity,' p. 112 (1913J. 

 t F. W. Aston, Phil. Mag. toI. xxxix. p. 449 (1920). 



