Penetration into glass 79 



when in contact with it. I should imagine, however, that it would still 

 be considerably overcharged. 



If one part of the plate is separated from the glass before the rest, as 

 must necessarily be the case, if it consists of bending materials, I should 

 guess it would be at least as much, if not more, overcharged, when sepa- 

 rated, as if it is separated all at once. 



In like manner, it should seem that the plate Ef will be considerably 

 undercharged, when separated from the glass, but not so much so as when 

 in contact with it. 



From the same kind of reasoning I conclude, that if the repulsion is 

 inversely as some lower power of the distance than the square, the plate Bd 

 will be considerably undercharged, and Ef considerably overcharged, when 

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

 in contact with it. 



X 34] 7- There is an experiment of Mr Wilcke and ^Epinus, related 

 by Dr Priestly, 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 communication between them, 

 a shock was felt like that of the Leyden vial. 



I am uncertain whether in this experiment the air contained between 

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

 undercharged 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 electricity 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. 



J 35] 8. Whenever any conducting body as A, communicating with 

 the ground, is brought sufficiently near an overcharged body B, the electric 



[* See Articles 344, 345, 511, 516.] 



