LAWS OF ELECTRIC PHENOMENA. 105 



of the t\vo kinds of electricity. This also was made by Dufay. 

 " Chance," says lie, " has thrown in ray way another principle raore 

 universal and remarkable than the preceding one, and which casts a 

 new light upon the subject of electricity. The principle is, that there- 

 are two distinct kinds of electricity, very different from one another ; 

 one of which I call vitreous, the other resinous, electricity. The first 

 is that of glass, gems, hair, wool, <fcc. ; the second is that of amber, 

 gam-lac, silk, &c. The characteristic of these two electricities is, that 

 they repel themselves and attract each other." This discovery does 

 not, however, appear to have drawn so much attention as it deserved. 

 It was published in 1735 ; (in the Memoirs of the Academy for 1733 ;) 

 and yet in 1747, Franklin and his friends at Philadelphia, who had 

 been supplied with electrical apparatus and information by persons in 

 England well acquainted with the then present state of the subject, 

 imagined that they were making observations unknown to European 

 science, when they were led to assert two conditions of bodies, which 

 were in fact the opposite electricities of Dufay, though the American 

 experimenters referred them to a single element, of which electrized 

 bodies might have either excess or defect. " Hence," Franklin says, 

 " have arisen some new terms among us : we say B," who receives a 

 .-park from glass, "and bodies in like circumstances, is electrized posi- 

 tin:hj\ A," who communicates his electricity to glass, "negatively; 

 or rather B is electrized plus, A minus" Dr. (afterwards Sir Wil- 

 liam) Watson had, about the same time, arrived at the same conclu- 

 sions, which he expresses by saying that the electricity of A was more 

 rare, and that of B more dense^ than it naturally would have been." 

 But that which gave the main importance to this doctrine was its appli- 

 cation to some remarkable experiments, of which we must now speak. 

 Electric action is accompanied, in many cases, by light and a crack- 

 ling sound. Otto Guericke 5 observes that his sulphur-globe, when rub- 

 bed in a dark place, gave faint flashes, such as take place when sugar 

 is crushed. And shortly after, a light was observed at the surface of 

 the mercury in the barometer, when shaken, which was explained at 

 first by Bernoulli, on the then prevalent Cartesian principles ; but. 

 afterwards, more truly by Hawkesbee, as an electrical phenomenon. 

 "YV all, in 1708, found sparks produced by rubbing amber, and Hawkes- 

 bee observed the light and the snapping, as he calls it, under various 

 modifications. But the electric spark from a living body, which, as 



4 Priestley, p. 115. 6 Exptrimcnta M<"^'.l'.>fgica, 1672, lib. iv. cap. 15. 



