44 Electric and Magnetic Science 



transfer some of A's electricity to the glass, whence it is 

 transferred to B. Thus A has a deficiency and B a superfluity 

 of electricity ; and if either of them approaches C, who has the 

 normal amount, the distribution will be equalized by a spark. 

 If, however, A and B are in contact, electricity flows between 

 them so as to re-establish the original equality, and neither is 

 then electrified with reference to C. 



Thus electricity is not created by rubbing the glass, but 

 only transferred to the glass from the rubber, so that the 

 rubber loses exactly as much as the glass gains ; the, total 

 quantity of electricity in any insulated system is invariable. This 

 assertion is usually known as the principle of conservation of 

 electric charge. 



The condition of A and B in the experiment can evidently 

 be expressed by plus and minus signs : A having a deficiency 

 - e and B a superfluity + e of electricity. Franklin, at the 

 commencement of his own experiments, was not acquainted 

 with du Fay's discoveries ; but it is evident that the electric 

 fluid of Franklin is identical with the vitreous electricity of 

 du Fay, and that du Fay's resinous electricity is, in Franklin's 

 theory, merely the deficiency of a stock of vitreous electricity 

 supposed to be possessed naturally by all ponderable bodies. 

 In Franklin's theory we are spared the necessity for admitting 

 that two quasi-material bodies can by their union annihilate each 

 other, as vitreous and resinous electricity were supposed to do. 



Some curiosity will naturally be felt as to the considerations 

 which induced Franklin to attribute the positive character to 

 vitreous rather than to resinous electricity. They seem to have 

 been founded on a comparison of the brush discharges from 

 conductors charged with the two electricities; when the 

 electricity was resinous, the discharge was observed to spread 

 over the surface of the opposite conductor " as if it flowed from 

 it." Again, if a Ley den jar whose inner coating is electrified 

 vitreously is discharged silently by a conductor, of whose pointed 

 ends one is near the knob and the other near the outer coating, 

 the point which is near the knob is seen in the dark to be illumi- 



