172 Dr. J. A. Fleming [May 30, 



or of so-called negative and positive ions and the strong attraction 

 between these involves the expenditure of work to separate them. 

 The radio-active substances, such as uranium, polonium, radium, 

 actinium and others, to which so much attention has been paid lately, 

 do seem to have the power of emitting their corpuscles or electrons 

 and scattering them abroad, and hence can only do this at the expense 

 of some of their own internal molecular energy or else drawing upon 

 the heat of surrounding bodies. 



We come next to the explanation of the familiar fact of electrifica- 

 tion by friction. Why is it that when we rub a glass rod with a bit 

 of silk the two things are equally and oppositely electrified ? To 

 explain this on the electronic theory we have to consider the state of 

 afiairs at the surface of any substance immersed say in air. At the 

 surface where the air and glass meet there will be an electronization 

 of atoms which appears to result in the formation of a double layer 

 of electrons and coelectrons or negative and positive ions. This is 

 probably an attempt on the part of the glass and air to combine 

 chemically together. The same state exists at the surface of the silk. 

 When we rub these two things together these double layers are very 

 roughly treated and are broken up. The whole lot of electrons and 

 coelectrons or residual portions of atoms get mixed up and more or 

 less divided up between the two surfaces. As however every negative 

 electron has its positive coelectrou, 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 

 signs. 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 neighbourhood of waterfalls of fresh water is electrified negatively, 

 whereas the air in the neighbourhood 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 au 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. 



