vol. LXI.] PHILOSOPHICAL TUANSACTIONS. 265 



repulsion is inversely as some lower power of the distance than the square, the 

 plate ud will be considerably undercharged, and e/ considerably overcharged, 

 when separated from the glass, but not in so great a degree as when they are in 

 contact with it. 



§ 7. There is an experiment of Mr. Wilke and ^pinus, related by Dr. 

 Priestley, p. 258, called by them, electrifying a plate of air: it consisted in 

 placing two large boards of wood, covered with tin plates, parallel to each other, 

 and at some inches asunder. If a communication was made between one of 

 these and the ground, and the other was positively electrified, the former was 

 undercharged; the boards strongly attracted each other; and, on making a com- 

 munication between them, a shock was felt like that of the Leyden vial. 



Mr. C. is uncertain whether, in this experiment, the air contained between 

 the two boards is very much overcharged on one side, and very much under- 

 charged on the other, as is the case with the plate of glass in the Leyden .vial ; 

 or whether the case is, that the redundant or deficient fluid is lodged only in the 

 two boards, and that the air between them serves only to prevent the electri- 

 city from running from one board to the other; but whichever of these is the 

 case, the experiment is equally conformable to the theory. 



It must be observed, that a particle of fluid, placed between the two plates, 

 is drawn towards the undercharged 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 being taken away, nearly in the ratio of twice the 

 quantity of redundant fluid actually contained in the plate, to that which it 

 would contain if electrified with the same force by itself; so that, unless the plate 

 is very weakly electrified, or their distance is very considerable, the fluid will be 

 apt to fly from one to the other, in the form of sparks. 



^ 8. Whenever any conducting body, as a, communicating with the ground, 

 is brought sufficiently near an overcharged body b, the electric fluid is apt to 

 fly 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 b towards a, and will consequently move slowly through 

 the air from one to the other: now it seems as if this motion increased the elas- 

 ticity 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 uninterrupted 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 body is negatively electrified, or in any other case in which one body is 



VOL. XIII. M M 



