244 POPULAR SCIENCE MONTHLY. 



for a certain time at least, and consequently accnmulatcd so as to 

 be observable; whereas the opposite electrification flowing into the 

 operator's hand continuously escaped to earth without giving any 

 sign of its presence. Had the operator stood upon an insulating sup- 

 port, the electrification would have accumulated on his body as well 

 as upon the sulphur. Guericke made the discovery that a light body, 

 having been once attracted to an electrified surface, was almost im- 

 mediately repelled from it, and could not be again attracted without 

 having its imparted electrification removed by contact with an un- 

 charged surface. 



Sir Isaac IsTewton, about 1675, made an interesting application 

 of a principle allied to this. He used a hollow, drum-shaped con- 

 trivance with glass ends and a very short axis, into which he put a 

 number of fragments of paper. On briskly rubbing the outside of 

 the glass with a piece of silk the paper was caused to " leap from one 

 part of the glass to another and twirl about in the air." Tliis was 

 repeated in 1676 before the Royal Society, to the great edification 

 of that learned body. 



Newton made a considerable improvement in the electrical ma- 

 chine of Guericke by the substitution of a hollow globe of glass for 

 Guericke's sulphur one. What is chiefly interesting about the im- 

 provement is the fact that Guericke's sulphur globe, of comparative 

 weight and cumbrousness, was made by casting melted sulphur into a 

 glass globe and then breaking off the glass. Guericke observed in 

 the dark a peculiar luminosity of conducting surfaces when well 

 charged by means of his machine; he compared it to the phospho- 

 rescent light observed when lump sugar is broken in the dark. It 

 was what is now known as the brush-discharge effect. 



In 1705 Francis Hawksbee discovered the peculiar phenomenon 

 which he termed the mercurial phosphorus. It was produced by 

 causing a stream of well-dried mercury to fall through an exhausted 

 glass receiver. The friction of the particles of mercury against the 

 jet piece and the glass caused an electrification which evinced itself 

 in a phosphorescent glow. The receiver, indeed, had not to be by 

 any means thoroughly exhausted, the phenomenon occurring at an 

 air pressure up to about fourteen inches of the barometer. 



The crackling noise and the spark accompanying electrical dis- 

 charge suggested about this time the analogy of those miniature dis- 

 turbances to thunder and lightning, but the identity of the two was 

 not fully established imtil later. 



Up to this time the fact that certain substances were capable of 

 conducting electricity was not known, but in 1729 Stephen Gray, 

 F, R. S., an enthusiastic investigator, made tlie discovery, and at the 

 same time the cognate one that a large class of materials are non- 



