CHARLES AUGUSTIN COULOMB 155 



Sixty years later again, Stephen Gray in London found 

 that electricity can be conducted; that there are conductors 

 and non-conductors of it. He carried out the conduction 

 of it over increasingly greater distances; in the year 1729, 

 by means of a string of hemp several hundred feet long and 

 supported by silk thread, he conducted it from a glass rod 

 electrified by friction to an ivory ball, at which the phenom- 

 ena of attraction could then be observed. From that time 

 it was possible to speak of the 'flow of electricity along con- 

 ductors,' and it was now more than ever described as a 

 'fluid.' Gray also was the first to set things, and also per- 

 sons, who were to be 'charged' with electricity, upon an 

 'insulating stool,' a cake of resin, and he recognised that a 

 hollow charged cube of wood behaves exactly like a solid 

 one of like size; from which it must be concluded, that 

 electricity has its seat only upon the surface, and not in the 

 whole interior of a body. Very noteworthy is also his ob- 

 servation of the non-interference between electrical and 

 magnetic forces; a key held up by a magnet could be insu- 

 lated and electrified, and attracted light test bodies, in just 

 the same way as in absence of the magnet. 



Not much later, in 1733, Dufay in Paris made the highly 

 important discovery that there are two kinds of electricity, 

 which behave in opposite ways, and were therefore called 

 positive and negative. He found that gold-leaf electrified 

 by glass, and hanging in the air, was not repelled, but at- 

 tracted, by electrified resin, and he followed this up closely; 

 but it was some time before his discovery was recognised. 



Ten years later a new observation was made. As the 

 electrical machine was gradually improved, it was fairly 

 natural for the attempt to be made to fill insulated bottles 

 with electricity, which was now available in plenty. Ewald 

 Jiirgen von Kleist, dean of a cathedral in Pomerania, and 

 later president of the High Court of Justice (and of the same 

 family as the poet Kleist) electrified a nail inserted in a 



