Ml 



ELECTRICITY. 



If* communication be now made between B3and 

 E f, by the canal NlfS. tin- redundant fluid Mill run 

 from B ) to E 9 ; and it' in h way it passes through the 

 body of any aniinnl. it will, by t'ir rapidity of it 

 tion, produce in it that spjisation culled .1 shock. 



It appears from the 22d proposition, that it' .. Nxly 

 of any siie was electrified in the same degree as tlic 

 pl.ite Rwanda communication was made between th it 

 body and tin- ground, by a canal of the same length, 

 breadth, and thickness, a- Mi> . th.tt then the fluid in 

 that canal would be impelled with the same force as 

 that in NHS, supposing the fluid in both canals to be 

 incompressible; and consequently, us the quantity of 

 fluid to be moved, mid the resistance to its motion, are 

 the same in both i-.in.-il-. the thud should move with the 

 same rapidity in both ; and there i- no re.'sou to think 

 that the case will be different, if the communication is 

 made by canals of real fluid. 



Therefore what was said in the beginning of this 

 Section, namely, that as great a shock would be produ- 

 ced by making a communication between the conductor 

 and the ground, as between the two sides of the I.ey- 

 den phial, by canals of the same length and same kind, 

 seems a necessary consequence of this theory ; as the 



rntity of fluid which passes through the canal is, by 

 supposition, the same in both ; and there is the 

 greatest reason to think, that die rapidity with which it 

 passes will be nearly, if not quite, the same in both. 



It may be worth observing, that the longer the canal 

 NHS is, by which the communication is made, the less 

 will l>e the rapidity with which the fluid moves along 

 it ; for the longer the canal is, the greater is the resist- 

 ance to the motion of the fluid in it ; whereas the 

 force with which the whole quantity of fluid in it is im- 

 pelled, is the same, whatever be the length of the canal. 

 Accordingly, it is found in melting small wires, by di- 

 recting a shock through them, that the longer the wire 

 the greater charge it requires to melt it. 



As the fluid in H 3 is attracted with great force by the 

 redundant matter in Y.Q, it is plain that if the fluid is 

 able to penetrate at all into the glass, great part of the 

 redundant fluid will be lodged in b 3 ; and In like man- 

 ner there will be a great deficiency of fluid in eq>. But 

 in order to form some estimate of the proportion of the 

 redundant fluid, which will be lodged in b 3, let the 

 communication between Ef and the ground be taken 

 away, as well as that by which B d is electrified ; and 

 let so much fluid In- taken from B 3, as to make the re- 

 dundant fluid in it equal to the deficient fluid in 1 * . If 

 we suppose that all the redundant fluid is collected in 

 6 3, and all the deficient in c Q, so as to leave B d and 

 _/ saturated ; then, if the electric repulsion is in- 

 vcr-ol v as the square of the distance, a particle of fluid 

 placed any where in the plane l> d, except near the ex- 

 tremities b and d, will be attracted with very near as 

 much force by the redundant matter in t f, as it is re- 

 pelled by the redundant fluid in 63. Hence it follows, 

 that if the depth to which the fluid can penetrate, is 

 very small in respect of the thickness of the glass, but 

 yet is such th.it the quantity of fluid naturally contain- 

 ed in A), or ftp, is considerably more than the redun- 

 dant fluid in B 3 ; then almost "all the redundant fluid 

 will be collected in A3, leaving the plate B</ not very 

 much overcharged ; and in like manner V.f will be not 

 very much undercharged. 



7. In the experiment of VVilke ami -T.pinus, called 

 electrifying a plate of air, (see page 528,) it does not 



PLATE 



r whether the ir contained between the two boards 

 i- very much ov< 1 on one side, and very much 



undereluri;. <1 on tle other, as is the case with the plate 

 of gl*S ii li :i phial ; or whether the ca.-e i-, that ,., 



ilundant or deficient fluid is lodged only in the two Wilke's 

 l-oard-, and that the air betw cm them serves only to prc- prrimcnt o 

 vent the electricity from running from one Ixuird to the * P'* te ' 

 other ; but whichever of th.-.-e is the ca-c, the expcri- *"' 

 ment is equally conformable to the tin 



It must lie observed, that a particle of fluid, placed 

 1 let ween the two plates, i- drawn towards the under- 

 charged plate, with a force exceeding that with which 

 it would be repelled from the overcharged plate, if it 

 was electrified with the same force, the other plate be- 

 ing taken away, nearly in the ratio of twice the quan- 

 tity of redundant fluid eontaintd in the plate, to that 

 which it would contain if electrified with the - 

 force by it-elf; so that, unless the plate is very weakly 

 electrified, or their distance very (l oiiiderable, the fluid 

 will fly fiom one to the other, in the form of sparks. 



8. Whenever any conducting body, as A, commu- Explanation 

 nicating with the ground, is brought sufficiently near ot the eltc- 

 an overcharged liody B, the electric fluid is apt to fly trie spark. 

 through the air from B to A, in the form of a spark": 

 the way by which this is brought about seems to be 

 this. The fluid placed any where between the two 

 bodies, is repelled from 1? towards A, and will conse- 

 quently move slowly through the air from one to the 

 other ; now it seems as if this motion increased the 

 elasticity of the air, and made it rarer ; this will enable 

 the fluid to flow in a swifter current, which will still 

 further increase the elasticity of the air, till at last it is 

 so much rarefied, as to form very little opposition to the 

 motion of the electric fluid, on which it flies in an un- 

 interrupted mass from one body to the other 



In the same manner may the electric fluid pass from 

 one body to another, in the form of a spark, if the first 

 body communicates with the ground, and the other 

 is negatively electrified, or in any other case in which 

 one body is strongly disposed to part with its electricity 

 to the air, and the other to receive it. 



In like manner, when the electric fluid is made to 

 through water, in the form of a spark, as in Beccaria's " 

 and Lane's t experiments, the water, by the rapid motion 

 of the electric fluid through it, is tunud into an elastic 

 fluid, and so much rarefied as to make very little op|K>- 

 sition to its motion ; and when stones are burst or thrown 

 out from buildings struck by lightning, that effect is 

 probably caused by the moisture in the stone, or some 

 of the stone itself being turned into an clastic fluid. 



It appears plainly, from the sudden rising of the wa- 

 ter in Kinnersley's electrical air thermometer, J that 

 when the electric fluid passes through the air, in the 

 form of a spark, the air in its passage is either very 

 much rarefied, or entirely displaced ; and the bursting 

 of the glass vessels, in Beccaria'a and Lane's experi- 

 ments, shows that the same thing happens with regard 

 to the water, when the electric fluid passes through it 

 in the form of a spark. Now there are no means by 

 which the displacing of the air or water can be brought 

 about, but by supposing its elasticity to be increased, 

 by the motion of the electric fluid through it, in 

 we suppose it to be actually pushed aside, by the force 

 with which the. electric fluid endeavours to issue from 

 the overcharged body ; but he can by no means think, 

 that the force with which the fluid endeavours to issue, 

 in the ordinary cases in which electric sparks are pro- 



* Elrttricitmo artijkialc t nnluroli, p. 110. Priestley, p. 209. 



f Pkil. Tram. 17B7, p. 451. ? Id. 1763, p. 84. Priestley, p. 210. 



