ELECTRICITY. 359 



the valve, and prevent any air from getting into 

 the exhausted glass. The inside of this phial re- 

 quires no coating, because, as the electric fluid 

 pervades a vacuum, it can pass freely from the wire 

 to the surface of the exhausted glass, without the 

 help of a non-electric coating. This phial exhibits 

 clearly the direction of the electric fluid, both in 

 charging and discharging; for if it be held by its 

 bottom, and its brass knob be presented to the 

 prime conductor positively electrified, you will see 

 that the electric fluid causes the pencil of rays to 

 proceed from the wire within the phial, as repre- 

 sented Fig. 21, and if it be discharged, a star will 

 appear in the place of the pencil, as represented in 

 Fig. 20. But if the phial be held by the brass cap, 

 and its bottom be touched by the prime conductor, 

 then the point of the wire on its side will appear 

 illuminated with a star when charging, and with a 

 pencil when discharging. If it be presented to a 

 prime conductor electrified negatively, all these 

 appearances, both in charging and discharging,, 

 will be reversed. 



To make the Electric Spark visible in Water* 



Fill a glass tube of about half an inch diameter, 

 and six inches long, with water, and to each extre- 

 mity of the tube adapt a cork to confine the water; 

 through each cork insert a blunt wire, so that the 

 extremities of the wire within the tube may be 

 very near one another: lastly, connect one of these 

 wires with the coating of a small charged phial, 

 and touch the other wire with the knob of it; by 

 which means the shock will pass through the wires, 

 and cause a vivid spark to appear between their 

 extremities within the tube. In performing this 



A A 4' 



