DISCOVERY BY MEANS OF NEW EXPERIMENTS. 533 



nent currency to the results of the labours of Gray and 

 others.' l He and Wheler also discovered, by means of 

 experiments, that the electric force might be transmitted 

 666 feet. In the year 1731, they found that the human 

 body is a conductor ; that sulphur, resins, &c., when 

 melted and allowed to cool upon insulating supports, were 

 electric ; and in 1732, that electricity may be retained in 

 bodies by wrapping them in worsted. Gray further dis- 

 covered by experiment, that substances in contact with 

 electrified bodies acquire the same electric properties. 



Du Fay, by means of suitable experiments, made 

 between the years 1733 and 1737, discovered that free 

 electricity is of two kinds, viz., vitreous, or that derived 

 by rubbing glass, and resinous, or that obtained by 

 friction of resin. He electrified a pith-ball (suspended by 

 silk) with a stick of electrified sealing-wax ; he then 

 rubbed the end of a glass rod with silk, and presented it 

 to the charged ball, and found that it was attracted, 

 whereas 'the charged sealing-wax repelled it ; and he thus 

 discovered the general truth, that similarly electrified 

 bodies repel each other, and dissimilarly electrified ones 

 mutually attract. It was previously known that any elec- 

 trified body attracts any non-electrified body, and that 

 after mutual contact they repel each other. In 1741 

 Boze found, by means of experiments, that electricity 

 does not alter the weight of bodies; and Ludolf, in 1744, 

 discovered that the electric spark will ignite ether and 

 other combustible bodies. Dr. Watson also, in 1747, by 

 suspending two miles of wire, and causing an electric 

 spark to pass through it, discovered that the rate of tra- 

 velling of electricity was practically instantaneous ; he 

 also found that the electric discharge would readily pass 

 through earth and water. 



1 Whewell, History of the Inductive Sciences, vol. iii. 3rd edit. p. P. 



