4j50 PHILOSOPHICAL TRANSACTIONS. [aNNO 1778. 



placed on the cake of resin excited with a considerable degree of electricity, will 

 not receive any electricity at all, or only a faint one, when it is separated from the 

 cake without having been touched when it was in contact with the cake, or in 

 the sphere of action of the cake, though it was real!) in a state of actual elec- 

 tricity all the time it was on the plate. Now, if the cake of resin did part as 

 easily with its state of electricity as the metal plate, it would leave a considerable 

 degree of electricity on the metal plate; the more as it is well known that the 

 metal does not at all resist the receiving of it. 



Though it would be perhaps in vain to attempt a further explanation of this 

 inherent quality of non-conducting bodies, yet it will be easy to illustrate this 

 law of nature by an example of another inherent quality in all matter, which 

 Sir Isaac Newton calls the vis inertias ; and is a vis insita, by which matter 

 resists being put in motion, and when it is once put in motion requires as much 

 force to stop its motion as it required to be brought from a state of rest to that 

 of motion. 



Let us now consider attentively the state of a body situated, as I have before 

 described, in the sphere of action of an excited electric ; as, for instance, a cake 

 of resin, a flat glass, or any other non-conducting substance ; or, hi other 

 words, let us consider the state of the metal plate placed on the resinous cake of 

 an electrophore, supposing tliis cake to be excited with a positive electricity ; 

 which electricity it acquires easily by sliding the knob of a Leyden phial, charged 

 in the common way, over its surface, and by various other ways. The super- 

 abundant electric fluid of the cake repels the electric fluid of the metal plate to 

 its farther extremity, and excites there an accumulation of that fluid ; or, in 

 other words, produces there a positive electricity, while it produces a negative 

 electricity at the surface in contact with the cake. If in this condition a con- 

 ducting body be brought in contact with the metal plate, or within its striking 

 distance, it receives a spark from it ; which spark is the electric fluid of the 

 metal plate crowded on the extremity of the metal by the repulsive force of the 

 superabundant electric fluid of the cake. If the metal plate be touched at that 

 side where it is really in a negative state, it will, notwithstanding, part with its 

 accumulated positive electricity ; because the repulsive power of the atmosphere 

 of the cake will force this crowded electric fluid out of whatever part of the 

 metal is touched, the electric fluid passing through metals very freely. 



The metal plate, thus deprived of the electric fluid crowded upon it, becomes 

 in a negative state ; but the repellent power of the electric fluid of the cake 

 continuing to act on the metal plate, forces what remains in it towards the far- 

 ther extremity, so as to produce much the same state as it had before it was put 

 on the cake ; so that the negative state, in which it is in reality, cannot appear 

 but when this metal is taken out of the pressing action of the atmosphere of the 



