A HISTORY OF SCIENCE 



tricity was produced, but if rubbed with flannel, resin- 

 ous electricity was produced. He discovered still 

 further that both kinds could be excited on the same 

 tube simultaneously with a single rubber. To demon- 

 strate this he used a tube, one-half of which had a 

 roughened the other a glazed surface. With a single 

 stroke of the rubber he was able to excite both kinds 

 of electricity on this tube. He found also that certain 

 substances, such as glass and amber, were electrified 

 positively when taken out of mercury, and this led to 

 his important discovery that an amalgam of mercury 

 and tin, when used on the surface of the rubber, was 

 very effective in exciting glass. 



