340 Mr. J. H. J. Poole on a Possible Connexion between - 



entirely on its chemical state of combination. This would 

 support the view that both the photo-electric and magnetic 

 classes of electrons in the atom of a ferro-magnetic element 

 are to be identified with the valency electrons, and hence we 

 might expect the act of magnetizing a body to have some 

 effect on its photo-electric properties. There are, never- 

 theless, two other possibilities with regard to the origin of 

 the photo-electron which might be suggested. These are 

 {a) that the photo-electron is emitted from the nucleus of 

 the atom, and (b) that it is a free or conductivity electron. 

 The first suggestion appears to be wildly improbable, as it is 

 only in radio-active changes that we usually find electrons 

 escaping from the nucleus. In this case the electron is 

 expelled with enormous velocity, and the number liberated 

 in a given time is totally unaffected by any external change 

 of physical or chemical conditions. The expulsion of such 

 an electron also leads to a change in the atomic number of 

 the atom, which entirely alters its chemical and physical 

 properties. The second alternative seems, ! owever, to be 

 quite reasonable, and would perhaps agree well with Prof. 

 Richardson's statistical theory of the emission of photo- 

 electrons, but we would expect that in this case the photo- 

 electric powers of an element would show some connexion 

 with its electrical conductivity, which is not borne out in 

 practice. Thus sulphur, which is nearly a perfect insulator,, 

 shows quite an appreciable photo-electric activity. Another 

 objection is that change of temperature, which largely affects 

 conductivity, has apparently no effect on the photo-electric 

 current from a metal in vacuo (Millikan & Winchester, Phil. 

 Mag. xiv. p. 188, 1907). On the other hand, the photo- 

 electric activity of an element runs exactly parallel with its 

 chemical properties, the more chemically electro-positive 

 the element is, the more active it is photo-electrically. 



We may then, I think, take it as highly probable that the 

 first supposition is true, and that the valency electron is 

 responsible for the photo-electric properties of an element. 

 While there is possibly not so much evidence that the 

 valency electrons are also the source of the magnetic powers,, 

 it seems quite likely that such is the case from the close 

 connexion between the chemical state of a magnetic element 

 and its maguetic properties, a fact which has already been 

 mentioned. Now, on Lindemann's Resonance Theory of the 

 emission of photo-electrons, as applied, at least, to the selec- 

 tive photo-electric effect, he assumes that the expulsion of 

 the electron by ultra-violet light is due to the perturbations- 

 produced by the light in the original orbit of the electron 



