104 GALVANIC ELECTRICITY. 



nected with the copper forms the positive electrode, and 

 the one attached to the zinc, the negative. 



250. Platinum wire is chosen, because 



Why is plati- 

 num used for there is frequent occasion to immerse the 



electrodes ? electrodes in corrosive liquids, and this me- 

 tal, for the most part, withstands their action. For 

 many experiments, it is found best to flatten the ends 

 of the wires forming the electrodes, so as to produce 

 a larger surface. The same object may also be effected 

 by terminating them with strips of platinum. 



251. ELECTRICAL CONDITION OF ATOMS. 



What is the ,, .. , . . 



electrical con- All atoms of matter are regarded as origi- 

 d atoms^ nally charged with either positive or nega- 

 tive electricity. Hydrogen and the metals 

 are electro-positive ; oxygen, chlorine, and cyanogen, 

 and other substances, to be described hereafter, are ne- 

 gative. A molecule of water is made up of a positive 

 atom of hydrogen, and a negative atom of oxygen ; 

 hydrochloric acid, of positive hydrogen and negative 

 chlorine ; oxide of silver, of positive silver and nega- 

 tive oxygen. The figure, in which +' represents 

 positive and negative, may represent a mole- 

 cule of either of the compounds named. 



252. QUANTITY OF ELECTRICITY. The 



What quanti- . 



ty of electrid- quantity of electricity thus combined or 

 Z i war t ? ned neutralized, in almost all kinds of matter, 

 is enormous. Faraday has shown that 

 a drop of water, contains more than is discharged in 

 the most violent flash of lightning. 



The terms atom t and molecule, .ire synonymous. But "molecule" is 

 limited, in the present work, to the'particle of a compound. 



