ELECTRONIC THEORY OF ELECTRICITY. 15 



two surfaces. As however every negative electron has its positive 

 coelectron, it follows that what one surface gains the other must lose. 

 Hence in the end we may have a majority of negative ions or electrons 

 left on the one surface and a majority of positive ions or coelectrons left 

 on the other surface, and the glass and the silk are then electrified with 

 equal quantities but opposite sign. Owing to the mutual repulsion of 

 the similar electrons the charge resides wholly on the surface. 



This conception of the existence of a double layer of opposite elec- 

 tricities or ions at the surface of contact of two substances has been put 

 forward to account for the familiar effect of the electrification of air by 

 falling drops of water. It has long been known that the air in the 

 neighborhood of waterfalls of fresh water is electrified negatively, 

 whereas the air in the neighborhood of splashing salt water, as at the 

 seaside, is positively electrified and the explanation that has been given 

 by Professor J. J. Thomson is that this is due to the breaking up of 

 this double layer of ions at the surface of the drop when it strikes the 



ground. 



Atomic Valency. 



At this stage it may be well to indicate that any valid theory of 

 electricity must involve an explanation of the facts of chemical com- 

 bination and chemical valency as well. At present all ideas on the 

 structure of atoms must necessarily be purely speculative. So much 

 advance has been made however in the development of a department 

 of chemistry called stereo-chemistry that we need not despair of coming 

 to know in time much about the architecture of atoms and molecules. 

 The way is cleared, however, for some consistent explanations if we can 

 assume that one or more free electrons can attach themselves to a 

 neutral atom and so give it a negative charge of electricity. We may 

 suppose as a first assumption that in a neutral atom which is otherwise 

 complete there exist localities at which one or more electrons can find 

 a permanent attachment. The atom is then no longer neutral but 

 negatively electrified. If the atom can as it were accommodate one 

 electron it is a monovalent element, if two it is divalent and so on. If 

 it cannot accommodate any at all it is an avalent or non-valent element. 



Consider the case of gaseous molecules. Chemical facts teach us. 

 that the molecules of free gaseous hydrogen, oxygen or other gases 

 contain two atoms, so that these free molecules are represented by the 

 symbols H,, 2 , etc. In these cases hydrogen and oxygen are so to 

 speak combined with themselves. We can explain this by the supposi- 

 tion that most neutral atoms are unstable structures. In contact with 

 each other some lose one or more electrons and an equal number gain 

 one or more electrons. Hence in a mass say of hydrogen we have some 

 atoms which are positively electrified and some which are negatively 

 electrified then called atomic ions, and these ions united pair and pair 



