noU'COfiJutf'ing Fo'ive*' of a perjcct Vacuum^ 275 



Qii the fluid entirely, without employing violence, which is 

 the cafe iii common and condenied air, but more particularly 

 in the latter. Thefe experiments, however, belong to another 

 fubject, and may poflibly be communicated at fome future 

 time. 



It is furprifing to obferve, how readily nn exhaufled tube is 

 charged with elecfhricity. By placing it at ten or twelve inches 

 from the condudlor the light may be feen pervading itsin{ide,and 

 as ftrong a charge may iometimes be procured as if it Vv'-ere in 

 conta6l with the conductor: nor does it iignify how narrovv^the 

 bore of the glafs maybe; for even a thermometer tube, having 

 the minutefl perforation poffible, will charge with the utmoil: 

 facility; and in this experiment the phasnomena are peculiarly 

 beautiful. 



Let one end of a thermometer tube be fealed hermetically. 

 Let the other end be cemented into a brafs cap with a valve, 

 or into a brafs cock, fo that it may be fitted to the plate of an 

 air-pump. When it is exhaufted, let the fealed end be applied 

 to the conductor of an electrical machine, while the other end 

 is either held in the hand or connected to the floor. Upon the 

 flighteft excitation the ele6tric fluid will accumulate at the fealed 

 end, and be difcharged through the infide in the form of a 

 fpark, and this accumulation and difcharge may be inceffantly 

 repeated till the tube is broken. By this means I have had a 

 fpark 42 inches long, and, had I been provided with a proper tube, 

 1 do not doubt but that I might have had a fpark of four times 

 that length. If, inftead of the fealed end, a bulb be blown at that 

 extremity of the tube, the ele6tric light will fill the whole of 

 that bulb, and then pafs through the tube in the form of a 

 brilliant fpark, as in the foregoing experiment ; but in this cafe 

 1 have feldom been able to repeat the trials above three or four 



N n 2 times 



