[ 33 ° 1 
other fmooth; it follows, that the power of eledtri- 
fying plus or minus arifes froth one and the fame fluid. 
Exp . 29. I then had a curiofity to try, whether I 
could not, by rubbing, make one fide of this glafs 
plus, and the other minus, at the fame time. This I 
effected, after both fides were made plus ; for, by rub- 
bing the rough fide of the glafs lefs, than I rubbed 
the fmooth fide, that became minus , and the fmooth 
fide continued plus. I rubbed the rough fide lefs, 
becaufe I found from experience, that rough glafs 
required a lefs power to electrify it minus , than 
Jmooth glafs did to eledtrify it plus ; and therefore I 
concluded, that the medium on the different furfaces 
has different powers , the great eft belonging to the 
fmooth, and the leaft to the rough furfaces ; as Sir 
lfaac Newton has fhewn concerning light falling up- 
on poliilied and rough glafs. 
I remember an obfervation of the like kind, which 
Mr. Short made, upon having occafion to heat one 
of his metal J'peculums , behind which was faftened a 
wooden handle. This Jpeculum he placed near a 
ftrong fire, with the polifhed furface towards the fame, 
where it continued above an hour, without receiving 
the leaft degree of warmth. That power therefore, 
which reflected the heat , mull certainly be of the fame 
nature with that, which occafioned the knobs of light 
in vacuo , mentioned in the firft experiment. I have 
frequently endeavoured to caufe alterations in that 
power, with a view to be better acquainted with its 
laws ; and, amongft other attempts, I rubbed elec- 
trics againft eledtrics. 
Exp. 30. The firft trial was with the Tourmalin 
and amber, which produced a plus eledtricity on both 
i 1 fides 
