VOL. LXX1I.] PHILOSOPHICAL TRANSACTIONS. '27 J 



plane, while the other electrified plate is on it, and afterwards separating them ; 

 for then both the metal plate and the conducting plane, which may be called the 

 inferior plane, will be found electrified, but possessed of contrary electricities, 

 as may be ascertained by electrometers. If the inferior plane be insulated first, 

 and then the electrified plate brought over it, then the latter will cause an endea- 

 vour in the former to acquire a contrary electricity, which however the insula- 

 tion prevents from taking place ; hence the intensity of the electricity of the 

 plate is not diminished, at least the electrometer will show a very little and almost 

 imperceptible depression, which small depression is owing to the imperfection of 

 the insulation of the inferior plane, and to the small rarefaction and condensa- 

 tion of the electric fluid, which may take place in different parts of the said in- 

 ferior plane. But if in this situation the inferior plane be touched, so as to cut 

 off the insulation for a moment, then it will immediately acquire the contrary 

 electricity, and the intensity in the metal plate will be diminished. 



If the inferior plane, instead of being insulated, were itself a non-conducting 

 substance, then the same phenomena would happen, viz. the intensity of the 

 electrified metal plate laid on it would not be diminished. This however is not 

 always the case ; for if the inferior non-conducting plane be very thin, and be 

 laid on a conductor, then the intensity of the electrified metal plate will be di- 

 minished, and its capacity will be increased by being laid on the thin insulating 

 stratum ; because in that case the conducting substance, which stands under the 

 non-conducting stratum, acquiring an electricity contrary to that of the metal 

 plate, will diminish its intensity, &c. and then the insulating stratum will only 

 diminish the mutual action of the two atmospheres more or less, according as it 

 keeps them more or less asunder. 



The intensity or electric action of the metal plate, which diminishes gradually 

 as it is brought nearer and nearer to a conducting plane not insulated, becomes 

 almost nothing when the plate is nearly in contact with the plane, the compen- 

 sation or accidental balance being then almost perfect. Hence, if the inferior 

 plane only opposes a small resistance to the passage of the electricity (whether 

 such resistance is occasioned by a thin electric stratum, or by the plane's imper- 

 fect conducting nature, as is the case with dry wood, marble, &c.) that resistance 

 joined to the interval, however small, that is between the two planes, cannot be 

 overcome by the weak intensity of the electricity of the metal plate, which on 

 that account will not dart any spark to the inferior plane (except its electricity 

 were very powerful, or its edges not well rounded) and will rather retain its elec- 

 tricity ; so that, being removed from the inferior plane, its electrometer will 

 nearly recover its former height. Besides, the electrified plate may even come 

 to touch the imperfectly conducting plane, and may remain in that situation for 



