14 ELEMENTS OF STATIC ELECTRICITY. 



the body with it, as in the descent of a weight or the 

 movement of water from a higher to a lower level. 

 But in the case of heat and electricity, the energy may 

 move while the body remains stationary ; and it may be 

 transferred from one body to another, or from one part 

 to another of the same body. Thus the mass of metal, 

 in the illustration given, transfers its heat energy to 

 another mass ; and in like manner, when a metal rod is 

 heated at one end, the heat moves to the cold end. 



Gravity apparently can move only by carrying the 

 body with it, while heat moves through the body with- 

 out producing change of position in its mass, like gravity. 



A hot body transfers its heat to a cold one in its 

 vicinity, but does not attract it ; while gravity produces 

 mutual attraction between all bodies, but is not trans- 

 ferred like heat from one body to another. 



But in electrified bodies we have both kinds of move- 

 ment. Like heat, electricity can move from one body 

 to another, or from one part to another of the same 

 body ; and, like gravity, it can carry the body with it. 



Hence we must distinguish between the movement of 

 electricity and the movement of the electrified body. 

 Electric movement, like heat 'movement, is from higher 

 to lower potential. If one part of a conductor be elec- 

 trified, the electricity instantly distributes itself over 

 every part. If two insulated bodies, free to move, are 

 placed in each other's vicinity, like the pith balls of the 

 electroscope, the same tendency to equilibrium is shown 

 by their mutual attraction. 



Though only one ball be electrified, yet it is evident 

 that their movement toward each other must be mutual, 

 and in proportion to their mass, since action and reac- 

 tion are equal : so that while the movement of the elec- 



