I 



of Elect ricUif. lOi 



this proof can hardlv be deemed conclusive, since it is, at 

 least, doubtful whether the wire and its coating ought to be 

 considered as in actual contact ; and if not so, a fluid so 

 extremely subtile might easily pass between them. That 

 the fluid mav be forced through the substance of wire is 

 more susceptible of proof from the instances of fusion; 

 but that its tendency is superficial may be ini'erred from \\ni 

 superior strength of yparks derived from conductors of large 

 surface, extended in length in preference to breadth, when 

 compared w ilh solid conductors ; — in short, it is found that 

 the prime conductor of an electrical machine is equally, if 

 not iijore, effectual when iioliow than when solid. In 

 some experiments made by Dr. Priestley, the excoriation of 

 metallic chains resulting from a strong discharge also de- 

 notes some determination towards the surface. 



If a perfect electric were in our possession, this hvpo- 

 thesis requires us to conceive of it as absolutely impermeable 

 to the electric fluid : but as the qualities of conducting and 

 non-conducting substances coalesce sometimes to a very ex- 

 traordinary degree, even in ulass, which we consider the best 

 electric — I must state that I mean by impermeabilitv, that 

 power of preventing the escape of the fluid which is pecu- 

 liar to electrics, which power is ascribed to the great diffi- 

 culty experienced by the electric fluid in obtaining admis- 

 sion into the pores of such bodies, and the extreme slow- 

 ness of its progressive motion over their surface or through 

 their substance. This power of confining the fluid is proved 

 by the charge of a Le\xlen phial, by the well-known in- 

 stances of msulation, and by the durability of electrical 

 properties. Mr. Henley (Phil. Trans, vol. Ixvii.) mentions 

 a small bottle which retained its electricity for seventy days 

 after charging, and remained during that time in an open 

 cupboard : he had a cylinder variable in the duration ot its 

 electricity : once, after excitation, it showed strong signs 

 of electricity for thirty-tlirec days : means were repeatedly 

 and successfully resorted to, to remove these appearances ; 

 but after a short pause they constantly returned without 

 fresh excitation, ah-:rnately became stronger and weaker, 

 vanished and returned without any visible cause : he ob- 

 served that the eieetricity was generally weak with a fire in 

 the room, or the door open ; during a northerly wind vigo- 

 rous : — the cylinder did not luiilormly retain this power; 

 it would often lose all signs of eieetricity in 12 hours after 

 excitation ; at other times it would remain a fortnight. 



ThediHerent re>-:ults of communicating eieetricity to con- 

 ductors, or electrics, afi'ord another proof of the difficult 



passage 



