1 1 6 Thoughts concerning Electricity 



of the electrified body may be much greater on that part of A which is 

 directly under it than on that which is farther removed from it, as is the 

 case in electrifying a prime conductor by an excited tube, then the case 

 is very different, for then on approaching the electrified tube, part of the 

 electric fluid will be driven away from that part of the prime conductor 

 which is nearest the excited tube to the remoter parts where its influence 

 is weaker, whereby that part of the conductor nearest the tube will be 

 undercharged, and consequently the compression of the electric fluid in 

 that part will be less than in the contiguous air, consequently some electric 

 matter will flow into it from the adjoining air, whereby the conductor 

 will be overcharged, and therefore on taking away the tube will be posi- 

 tively electrified. 



Thus if the excited tube or other electrified body is not brought within 

 a certain distance, the conductor receives its electricity only from the 

 contiguous air, as was before said, and not immediately from the electrified 

 body; but if the body be brought near enough, the electric matter jumps 

 from the electrified body to the conductor in form of a spark. 



212] The means by which this is brought about seems thus When 

 the part of the conductor nearest the excited tube has received any 

 electricity from the contiguous air, that air will be undercharged, and will 

 receive electricity from the adjacent air between it and the tube, by which 

 means the electric matter will flow in gentle current between the particles 

 of air from the excited tube to the conductor. It seems now as if the 

 particles of air were by this means made to repel each other with more 

 force, and thereby to become rarer; this will suffer the electric fluid to 

 flow in a swifter current, which again will increase the repulsion of the 

 particles of air, till at last a vacuum is made, upon which the electric fluid 

 jumps in a continued body to the conductor. 



213] That a vacuum is formed by the electric fluid when it passes in 

 the form of a spark through air or water appears, I think, from the violent 

 rising of the water in Mr Kinnersley's electrical air-thermometer (Priestley, 

 p. 216), and still more strongly from the bursting the vial of water, in 

 Mr Lane's experiment, by making the electrical fluid pass through the 

 water in the form of a spark. 



If I am not much mistaken I have frequently observed, in discharging 

 a Leyden vial, that if the two knobs are approached together very gently, 

 a hissing noise may be perceived before the spark, which shews that the 

 electricity does begin to flow from one knob to the other before it moves 

 in the form of the spark, and may therefore induce one to think that the 

 spark is brought about in the gradual manner here described. 



214] The attraction and repulsion of electrified bodies, according to 

 the law I have laid down, may perhaps be accounted for in the following 



