2 PROPERTIES OF AN ELECTRIC CHARGE [CH. I. 



represent all that is observed as electrical attractions 

 and repulsions. 



The field of force mapped by the lines can exist 

 in vacuo perfectly well, but the lines never terminate 

 in vacuo ; the charges are always carried by matter, 

 or by something equivalent thereto. In empty space 

 it is probable that the only way of destroying such a 

 field of force is to allow the two bodies possessing the 

 charges to approach each other, and thus shorten up 

 the lines to nothing ; though, even so, it is not prob- 

 able that the charges are destroyed, but only placed 

 so close together that they have no external effect 

 at any moderate distance. When matter is present, 

 however, it may be able to assist this collapse of the 

 lines in various ways, giving rise to the various 

 phenomena of conduction and of disruptive discharge. 



If one of the two oppositely charged bodies is sent 

 away to a considerable distance, while the other is 

 isolated and regarded alone, the lines of this latter 

 start out in all directions in nearly straight lines, 

 giving rise to the simple notion of a single charged 

 body. There must however be a complementary 

 charge, the other ends of the lines must be some- 

 where ; though they may be so far away as to be 

 spoken of as, for all practical purposes, at infinity. 



Parenthetical remark of general application. 

 People often feel hesitation about the treatment of 

 things as at infinity, as if it introduced a conception 

 of some difficulty ; but they should realise that this 

 mode of expression is always employed as a simplifi- 

 cation, whenever it happens that for present purposes 

 the said things can be ignored. If their existence 

 requires attention, it must be recognised that they 

 are really at some finite distance, and their location 

 must be specified ; but such specification complicates 



