Chap. 2.] Inftwments ujed in Ekfirkity. 315 



turn round , thirdly, metallic conductors, or fubftanees 

 charged with humidity; fourthly, electric bottles, 

 commonly called Leyden phials j fifthly, eledric bat- 

 teries. 



The firft electrical machine made ufe of was a tube 

 of glafs, which, being electrified by friction, was then 

 put in a (late to communicate electricity to other bo- 

 dies. The beft glafs for this purpofe is the fine white 

 Englilh cryftal. The moil convenient dimerifions for 

 thefe tubes are about three feet of length, twelve or 

 fifteen lines of diameter, and quite a line of thicknefs. 

 It is of little importance whether the tube is open or 

 clofed at the extremities ; yet it is necefTary that the 

 air within fhould be in the fame ftate as that without j 

 for this reafon the tube fhould at lean: be open at one 

 end j but care muft be taken left dirt fhould be ad- 

 mitted into the infide, for that would confiderably im- 

 pede its effects. If, notwithftanding thefe precau- 

 tions, the tube receives either dirt or moifture, fome 

 dry and fine fand fhould be introduced into the infide, 

 and it fhould be afterwards cleaned out with fine dry 

 cotton. 



When it is intended to electrify a tube, it is only 

 neceffary to take the end in one hand, and to continue 

 to rub the tube with the other hand from one end to 

 the other until it affords marks of its being fufficiently 

 charged with the electric fluid. This friction may be 

 performed with the naked hand when it is dry and 

 clean, otherwife with a piece of brown paper, or wax- 

 ed taffeta. When .the tube has been rubbed in this 

 manner, the circumambient air being dry, if light fub- 

 ftanees are prefented to it, they will be firft attracted 

 towards it and immediately afterwards repelled. 



The electric fluid may be excited in nearly a fimi- 



lar 



