Chap, i.] THEORIES OF ELECTRICITY. 3 



supposes that there are two kinds of electricity, is due 

 to the French academician, Dufay, and was afterwards 

 worked out by Robert Symmer. It supposes that 

 all bodies have a certain amount of both, which 

 equalise one another, so that the body appears im- 

 electrified. The body may, however, gain an excess 

 of one of the fluids, and as the total amount is always 

 the same, it loses a corresponding quantity of the 

 other, and then appears electrified by the fluid which 

 is in excess. 



Neutralisation of one electricity by the other may 

 be shown by touching a pith-ball, previously charged 

 from a glass rod, by an electrified stick of resin. The 

 ball at once ceases to be electrified. 



The one-fluid tlieory supposes only one kind of 

 electricity, of which all unelectrified bodies have a 

 normal amount. Bodies may, however, be caused to 

 have more than the normal amount when they are 

 said to be positively electrified ; or they may have less 

 than the normal amount when they are negatively 

 electrified. This is the theory propounded by 

 Franklin in 1747. 



On both theories the fluids, or fluid, are mobile and 

 imponderable, and permeate all ponderable matter. 



Franklin's phraseology is generally adopted, though 

 not his theory, positive and negative being convenient 

 terms for designating the electrical state of bodies. 



o o 



Positive is equivalent to vitreous, and negative to 

 resinous, the one being often signified by the sign + , 

 and the other by the sign . 



When the glass becomes positively electrified by 

 friction, the rubber is found to be negatively electrified ; 

 and while the resin is negatively electrified, the skin 

 with which it is rubbed is positively electrified. This 

 is often difficult to show, because the electricity of the 

 rubber may be conducted away through the body to 

 the ground. 



