622 SCIENCE PROGRESS 



to observe the production of neon or helium by electric dis- 

 charge : his results were negative. 



In connection with J. J. Thomson's work on the positive rays, 

 which seem to offer a new and very sensitive method of chemical 

 analysis, it may be mentioned that Moseley found that the X-ray 

 spectra already described gave good evidence of traces of foreign 

 metals in some of the metals used, the strong line in the spectrum 

 of the impurity appearing distinctly along with the spectrum of 

 the metal itself. The X-ray spectrum, being apparently so much 

 simpler in character than the ordinary spectra, may in this way 

 afford a very powerful method of chemical analysis. 



Two very interesting papers have appeared which trace effects 

 previously assumed to be of purely electronic origin to the pre- 

 sence of small quantities of gas. The effects in question are the 

 photoelectric effect and the thermionic effect, which have been 

 the object of so much study in recent years, and concerning 

 which so many inconsistent results have been obtained. From 

 a careful study of the behaviour of purified carbon Pring has 

 come to the conclusion that the emission of electrons by this 

 substance ordinarily observed when it is raised to a high tem- 

 perature is due largely, if not entirely, to the presence of traces 

 of gas. He has shown that by very carefully purifying the 

 carbon and reducing the pressure of the surrounding gas as 

 much as possible the discharge of negative electricity can be 

 diminished to an enormous extent, that on the admission of a 

 little gas the thermionic current gradually increases correspond- 

 ing to the occlusion of the gas, and that the effect depends in a 

 high degree on the nature of the traces of residual gas, being 

 very small indeed with the inactive gases helium and argon, and 

 relatively considerable with carbon dioxide, which is known to 

 react with carbon at high temperatures. These and other results 

 furnish a strong presumption that the so-called thermionic effect 

 is due to the presence of traces of gas, which react, probably in 

 a cyclic process, with the carbon, the reaction resulting in the 

 liberation of electrons ; the effect is thus probably a chemical 

 one. Freydenhagen came to similar conclusions in 191 2 with 

 respect to sodium and potassium in a high vacuum, after Pring 

 had already published preliminary results on carbon. It remains 

 to be seen if there is a residual true thermionic effect, which 

 must be in any case very small, and if the effect in different 

 metals is due to traces of gas. Considered in conjunction with 



