Positive and negative electrification 67 



opinion, that excited glass contains more than its natural quantity of 

 electricity) ; even though some of them, by the approach of a stronger 

 electrified body, are made undercharged. I shall use the words in the 

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

 shall use them more accurately, I shall give the following definition. 



102] In order to judge whether any body, as A, is positively or nega- 

 tively 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 under- 

 charged body; and let B contain the same quantity of electric fluid as if 

 it communicated with A by a canal of incompressible fluid : then, if B is 

 overcharged, I call A positively electrified; and if it is undercharged, I 

 call 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. 



103] It appears from the corollary to the 24th proposition, that if 

 several bodies are insulated, and connected together by conducting sub- 

 stances, and one of these bodies is positively or negatively electrified, all 

 the other bodies must be electrified in the same degree: for supposing a 

 given body B to be placed at an infinite distance from any over or under- 

 charged body, and to contain the same quantity of fluid as if it com- 

 municated with one of those bodies by a canal of incompressible fluid, all 

 the rest of those bodies must by that corollary contain the same quantity 

 of fluid as if they communicated with B by canals of incompressible fluid : 

 but yet it is possible that some of those bodies may be overcharged, and 

 others undercharged: for suppose the bodies to be positively electrified, 

 and let an overcharged body D be brought near one of them, that body 

 will become undercharged, provided D is sufficiently overcharged; and yet 

 by the definition it will still be positively electrified in the same degree 

 as before. 



Moreover, if several bodies are insulated and connected together by 

 conducting substances, and one of these bodies is electrified by excited 

 glass, there can be no doubt, I think, but what they will all be positively 

 electrified ; for if there is no other over or undercharged body placed near 

 any of these bodies, the thing is evident ; and though some of these bodies 

 may, by the approach of a sufficiently overcharged body, be rendered 

 undercharged ; yet I do not see how it is possible to prevent a body placed 

 at an infinite distance, and communicating with them by a canal of in- 

 compressible fluid, from being overcharged. 



In like manner if one of these bodies is electrified by excited sealing 

 wax, they will all be negatively electrified*. 



104] It is impossible for any body communicating with the ground 

 to be either positively or negatively electrified: for the earth, taking the 



[* Note 7, p. 372.] 



52 



