14 POPULAR SCIENCE MONTHLY. 



sudic chloride, is placed in water a certain proportion of the molecules 

 of sodic chloride are dissociated into sodium and chlorine ions, that is 

 to say, atoms possessing electric charges, and the electric conductivity 

 of the solution is due to the mobility of these saline ions. 



On the electronic theory a certain proportion of the atoms of a 

 conductor are similarly in a state of electronization. The application of 

 an electromotive force to the conductor thus at once causes the electrons 

 to begin to migrate. If we compare conductors and non-conductors 

 we shall see that the former are mostly elementary bodies, the metals 

 and alloys or graphitic carbon, whilst the latter are all very complex 

 substances such as glass, ebonite, the oils, shellac, gutta-percha, etc. 

 These last have large and complex molecules but the good conductors 

 have all simple molecules and small atomic volumes. The exceptions 

 apparently are sulphur and carbon in the form of diamond. When, 

 however, we remember that carbon and sulphur are elements very prone 

 to polymerize and so to speak combine with themselves they may not 

 really be an exception. The electrons may therefore have much more 

 difficulty in exchanging from atom to atom or in making their way 

 between or through the molecules when these are very complex than 

 when they are simple. 



The question then may be asked why these free electrons do not all 

 escape from the conductor. The answer is that there must be an equal 

 quantity of electrons and coelectrons or remainders of atoms or of so- 

 called negative and positive ions, and the stronger 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 sur- 

 rounding 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 affairs 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 por- 

 tions of atoms get mixed up and more or less divided up between the 



