Exchange of electricity with the air 71 



115] CASES X and XI. By the same reasoning it appears, that if the 

 two bodies are both negatively electrified in the same degree, they must 

 repel each other: but if they are both negatively electrified in different 

 degrees, it is possible for them to attract each other. . 



All these cases are exactly conformable to experiment. 



116] CASE XII. Let two cork balls be suspended by conducting 

 threads from the same positively electrified body, in such manner that if 

 they did not repel, they would hang close together: they will both be 

 equally electrified, and will repel each other : let now an overcharged body, 

 more strongly electrified than them, be brought under them; they will 

 become less overcharged, and will separate less than before: on bringing 

 the body still nearer, they will become not at all overcharged, and will 

 not separate at all : and on bringing the body still nearer, they will become 

 undercharged, and will separate again. 



117] CASE XIII. Let all the air of a room be overcharged, and let 

 two cork balls be suspended close to each other by conducting threads 

 communicating with the wall. By Prop. 15, it is highly probable that the 

 balls will be undercharged; and therefore they should repel each other. 



These two last cases are experiments of Mr Canton's, and are described 

 in Philosophical Transactions 1753, p. 350, where are other experiments 

 of the same kind, all readily explicable by the foregoing theory. 



I have now considered all the principal or fundamental cases of electric 

 attractions and repulsions which I can think of ; all of which appear to 

 agree perfectly with the theory*. 



118] 3. On the cases in which bodies receive electricity from or part 

 with it to the air. 



LEMMA I. Let the body A (Fig. 6) either stand near some over or 

 undercharged body, or at a distance from any. 

 It seems highly probable, that if any part of 

 its surface, as MN, is overcharged, the fluid 

 will endeavour to run out through that part, 

 provided the air adjacent thereto is not over- 

 charged. 



For let G be any point in that surface, and 

 P a point within the body, extremely near to it ; 

 it is plain that a particle of fluid at P must be repelled with as much force 

 in one direction as another (otherwise it could not be at rest) unless all the 

 fluid between P and G is pressed close together, in which case it may be 

 repelled with more force towards G than it is in the contrary direction : now 

 a particle at G is repelled in the direction PG, i.e. from P to G, by all the 

 redundant fluid between P and G ; and a particle at P is repelled by the 



[* Note 8, p. 373.] 



