VOL. LXI.] PHILOSOPHICAL TRANSACTIONS. 25S 



As to the fluid contained in non-conducting substances, though it does not ab- 

 solutely answer to the definition of immoveable, as it is not absolutely confined 

 from moving, but only does so with great difficulty ; yet it may in most cases be 

 considered as such without sensible error. Air does in some measure permit the 

 electric fluid to pass through it ; though, if it is dry, it lets it pass but very 

 slowly, and not without difficulty ; it is therefore to be called a non-conductor. 



It appears that conductors would readily suffer the fluid to run in and out of 

 them, were it not for the air which surrounds them : for if the end of a conduc- 

 tor is inserted into a vacuum, the fluid runs in and out of it with perfect readi-^ 

 ness ; but when it is surrounded on all sides by the air, as no fluid can run out 

 of it without running into the air, the fluid will not do so without difficulty. If 

 any body is surrounded on all sides by the air, or other non-conducting sub- 

 stances, it is said to be insulated : if on the other hand it any where communi- 

 cates with any conducting body, it is said to be not insulated. When he says 

 that a body communicates with the ground, or any other body, he would be 

 understood to mean that it does so by some conducting substance. 



Though the terms positively and negatively electrified are much used, yet the 

 precise sense in which they are to be understood, seems not well ascertained ; 

 namely, whether they are to be understood in the same sense in which he has 

 used the words over or undercharged, or whether, when any number of bodies, 

 insulated and communicating with each other by conducting substances, are 

 electrified by means of excited glass, they are all to be called positively electrified 

 (supposing, according to the usual opinion, that excited glass contains more 

 than its natural quantity of electricity) ; even though some of them, by the ap- 

 proach of a stronger electrified body, are made undercharged. He uses the 

 words in the latter sense; but as it will be proper to ascertain the sense in which 

 he uses them more accurately, he gives the following definition. In order to 

 judge whether any body, as A, is positively or negatively electrified: suppose 

 another body b, of a given shape and size, to be placed at an infinite distance 

 from it, and from any other over or undercharged body ; and let b contain the 

 same quantity of electric fluid, as if it communicated with a by a canal of in- 

 compressible fluid : then if b is overcharged, he calls a positively electrified ; 

 and if it is undercharged, he calls a negatively electrified; and the greater the 

 degree in which b is over or undercharged, the greater is the degree in which a 

 is positively or negatively electrified. 



It appears from the corol. to the 24th proposition, that if several bodies are 

 insulated, and connected together by conducting substances, and one of these 

 bodies is positively or negatively electrified, all the other bodies must be electri- 

 fied in the same degree : for suppose a given body b to be placed at an infinite 

 distance from any over or undercharged body, and to contain the same quantity 



