ELECTRONIC THEORY OF ELECTRICITY. 17 



for neutral atoms. Thus we may consider that the metallic atoms lose 

 very easily one or more electrons and also that there is a somewhat feeble 

 attachment in their case between the neutral atom and the free electron. 

 Hence metals in the mass are conductors because there are plenty of 

 free electrons present in them. On the other hand, in the case of non- 

 metallic atoms the force required to detach one or more electrons from 

 the atom is much greater and conversely the attachment of free elec- 

 trons for the neutral atom is larger. Accordingly, in non-metals 

 there are few free electrons and they are therefore non-conductors. 

 Moreover the presence of positive and negative atomic ions causes 

 them to link together into more or less complex molecules and they 

 exhibit polyvalency and act as the grouping elements in molecular com- 

 plexes. This is a very characteristic quality of the elements, sulphur, 

 silicon and carbon. 



Helmholtz long ago laid stress on the fact that certain physical and 

 chemical effects could only be explained by assuming a varying attrac- 

 tion of electricity for matter. The same idea followed out leads to an 

 hypothesis of chemical combination and dissociation of salts in solu- 

 tion. Thus a molecule of sodic chloride is the electrical union of a 

 monovalent sodium ion or sodium atom minus one electron with a 

 chlorine ion which is a chlorine atom plus one electron. It may be 

 asked why in this case does not the extra electron pass over from the 

 chlorine to the sodium ion and leave two neutral atoms. The answer 

 is because the union between the electron and the chloride is probably 

 far more intimate than that between the atomic groups. These latter 

 may revolve round their common center of mass like a double star, but 

 the electron which gives rise to the binding attraction may be more 

 intimately attached to the atomic group into which it has penetrated. 



Voltaic Action. 



Any theory of electricity must in addition present some adequate 

 account of such fundamental facts as voltaic action and magneto- 

 electric induction. Let us briefly consider the former. Suppose a strip 

 of copper attached to one of zinc and the compound bar immersed in 

 water to which a little hydrochloric acid has been added. 



All chemical knowledge seems to point to the necessity and indeed 

 validity of the assumption that the work required to be done to remove 

 an electron from a neutral atom varies with the atom. Conversely the 

 attraction which exists between a free electron and an atom deprived 

 of an electron also varies. Accordingly the attraction between atomic 

 ions, that is, atoms one of which has gained and one of which has lost 

 electrons, is different. Upon this specific attraction of an atomic ion 

 for electrons or their relative desire to form themselves into neutral 

 molecules depends what used to be called chemical affinity. Mr. Euther- 



VOL. lxi. — 2. 



