VOL. LI.] PHILOSOPHICAL TRANSACTIONS. 401 



Exp. 26. The tourmalin aftbrded like appearances when in the same circum- 

 stances; with this difference, that they were caused at greater distances than 

 those of the glass; and particularly the plus electricity was acquired at the dis- 

 tance of one foot or more. 



Exp. 27. Having by him a pane of glass, one side of which was rough, and 

 the other smooth, he rubbed it slightly on the rough side ; on which both sides 

 were electrified minus. 



Exp. 28. He treated the other side in the same manner; after which the 

 minus electricity was changed to a plus one on both sides. 



Exp. 29. He then tried whether he could not by rubbing make one side of 

 this glass plus, and the other minus at the same time. This he effected, 

 after both sides were made plus ; for by rubbing the rough side of the glass less 

 than the smooth side, that became minus, and the smooth side continued plus. 

 He rubbed the rough side less, because he found from experience that rough 

 glass required a less power to electrify it minus, than smooth glass did to elec- 

 trify it plus; and therefore he concluded, that the medium on the different sur- 

 feces has different powers, the greatest belonging to the smooth, and the least 

 to the rough surfaces; as Sir Isaac Newton has shown concerning light falling 

 on polished and rough glass. 



Mr. W. remembered an observation of the like kind, which Mr. Short made 

 on having occasion to heat one of his metal speculums, behind which was fast- 

 ened a wooden handle. This speculum he placed near a strong fire, with the 

 polished surface towards the same, where it continued above an hour, without 

 receiving the least degree of warmth. That power therefore which reflected the 

 heat, must certainly be of the same nature with that which occasioned the knobs 

 of light in vacuo, mentioned in the first experiment. Mr. W. had frequently 

 endeavoured to cause alterations in that power, with a view to be better acquainted 

 with its laws; and among other attempts he rubbed electrics against electrics. 



Exp. 30. The first trial was with the tourmalin and amber, which produced a 

 plus electricity on both sides of the stone, and a minus one in the amber. 

 Afterwards he electrified the amber, and held it near the tourmalin ; still both 

 sides were plus : and if he rubbed the tourmalin while the amber was electrified, 

 it continued plus. Then he rubbed the stone with glass : yet both sides of the 

 tourmalin were plus, and the glass minus. 



Exper. 3 1 . But when the glass was electrified plus, and held near the tour- 

 malin, as had been done before with the amber, in this case, both sides were 

 electrified minus. 



These experiments seem to show, that where electric appearances are pro- 

 duced by the rubbing of any two polished bodies together, that body whose sub- 

 stance is hardest, and electric }x:)wer strongest, will be always plus, and the 



VOL. XI. 3 F 



