CHAPTER V. 



THEORY OF ELECTRICITY. 



In the series of chapters on Heat (Vol. II) 

 and in the chapter on Magnetism the word 

 molecule was frequently used synonymously 

 with atom. In chemistry a distinction is 

 made, and as we can better explain the theory, 

 at least, of electricity by keeping this distinc- 

 tion in mind we will refer to it here. 



Tt has been stated that there are between 

 sixty and seventy elementary substances. An 

 elementary substance cannot be destroyed as 

 such. It can be united with other elements 

 and form chemieal compounds of almost end- 

 less variety. The small< ->t particle of an ele- 

 mentary substance is called in chemistry an 

 atom. The smallest particle of a compound 

 substance is called a molecule. The atom is 

 the unit of the element, and the molecule is 

 the unit of the compound as such. It follows, 

 then,, that there are as many different kinds of 

 atoms :i< there are elements, and as many 

 diiTerent kinds of molecules as there are com- 

 pound-. If the element- have a molecular 

 structure then two ,,r more atoms of the same 



