540 Sir J. J. Thomson on the Structure of 



which, if its structure were interpreted in terms of the 

 ordinary theory of valency, would contain no double bonds, 

 i.e. it would be a saturated compound; such a molecule 

 would on this theory be incapable of receiving a negative 

 charge, whereas if it contain double or triple bonds some of 

 the cells would have more than two electrons in common, and 

 by opening out this contact might be able to accommodate 

 more electrons and thus receive a negative charge. 



The negative charge on the molecule indicates that the 

 contact or, in the usual terminology, the double bond has 

 been loosened ; so that it would seem possible that an effective 

 way of loosening this bond, i.e. turning unsaturated into 

 saturated compounds, would be to give the molecules a 

 negative charge by exposing them to a stream of electrons. 

 This may play a part in the Sabatier-Senderens method of 

 reducing unsaturated compounds bypassing them over finely 

 divided metals at a high temperature. The hot metal is 

 well known to be a source of electrons. 



Let us consider from this point of view the formation of 

 negative ions in another case — that of a gas ionized by 

 Rontgen rays. We may suppose that the first effect of these 

 rays is to eject electrons from the molecules, so that initially 

 the negative ions are electrons. They will remain electrons, 

 and so have much greater mobility than the positive ions, 

 unless they can attach themselves to atoms or molecules. 

 But if the gas, like neon or argon, consists of atoms with 

 8 electrons in the outer layer there is no room for the 

 electron on the atom, and if it collides with the atom it will 

 rebound and remain a free electron — the mobility of the 

 carriers of negative electricity will be that of an electron, 

 and will far transcend that of the positive ion. Franck and 

 Hertz long ago called attention to the great mobility of the 

 negative ion in argon. Next suppose that the gas is not 

 monatomic, but that in the molecule there is only a single 

 octet of electrons, as in N 2 and CO ; in this case, again, there 

 is no room for an electron, and we should expect the electron 

 to remain free and have a high mobility. Franck and Hertz 

 have shown that this is true for nitrogen. Suppose, however, 

 that the molecule, like those of oxygen or chlorine, contains 

 two octets with some electrons in common; theu, by opening 

 up the contacts the molecule could accommodate more elec- 

 trons, so that in this case the electron could attach itself to 

 the molecule and thereby make its mobility comparable with 

 that of the»positive ion. With oxygen, where the octets 

 have four electrons in common, the opening of the contacts 

 might occur without the separation of the atoms. In chlorine, 

 however, where the octets have only two electrons in common, 



