WHAT IS ELECTRICITY ? 197 



as sodion ; that is, sodium less an electron. The chlorine 

 is in the water, not as chlorine ; by gaining an electron, it 

 has been converted into chlorion. We see, therefore, that 

 those elements which we call metals become ions by 

 losing electrons; while those which we call non-metals 

 become ions by gaining electrons. 



Let us now consider the simple battery or cell, consist- 

 ing of a plate of copper and a plate of zinc, dipping in a 

 jar half full of dilute hydrochloric acid. This hydro- 

 chloric acid consists of a number of ions of hydrogen; 

 and ions of hydrogen differ from ordinary hydrogen gas 

 in the same way as ions of sodium differ from metallic 

 sodium, namely, by each atom having parted with an 

 electron. The electron which each atom has lost has 

 attached itself to an atom of chlorine, and the chlorine 

 atom is thereby converted into an ion. 



The plate of zinc cannot dissolve in the water, until its 

 atoms have been converted into ions. They would then 

 each have to part with two electrons. But the attraction 

 of an atom of zinc for these two electrons is so great that 

 the zinc does not dissolve, unless, indeed, the electrons 

 can be conveyed elsewhere. 



Now electrons have the power of travelling through 

 metal; this point will be considered later; it must be 

 accepted for the present. When an atom of zinc gives up 

 its two electrons to the zinc plate, the atom of zinc which 

 lies nearest to that which has parted with these two elec- 

 trons will be overloaded ; it already is in combination with 

 its own two, and cannot unite with two additional ones ; 

 or, if it does, it must pass on its own electrons to the 

 neighbouring atom. 



These two electrons, therefore, displace others, or, it may 

 be, are themselves transmitted through the zinc, until 

 they reach the copper wire. Copper, in the metallic 

 state, is also a compound of copper ions with two elec- 



