C 782 ] 
But if the rough tube be greafed all over with 
tallow from a candle, and as much as poflible of it 
wiped off with a napkin, then the oiled filk will 
receive a kind of polifh bv rubbing it, and after a 
few ftrokes, will make the tube ad: in the fame 
manner as when excited at firff by flannel. 
The oiled filk, when covered with chalk, or 
whiting, will make the greafed rough tube ad again 
like a polffhed one : but if the fridion be continued 
till the rubber is become very fmooth, the eledric 
power will be changed to that of fulphur, fealing- 
wax, &c * 
Thus may the pofitive and negative powers of 
cledricity be produced at pleafure, by altering the 
furfaces of the tube and rubber ; according as the one 
or the other is mod affeded by the fridion between 
them : For if the polifli be taken ofF one-half of a 
tube, the different powers may be excited with the 
fame rubber at a Angle ffroke. And the rubber is 
found to move much eafler over the rough, than 
over the polifhed part of it. 
That polifhed glafs eledrifcs pofltively, and rough 
glafs rubbed with flannel negatively, feems plain, 
from the appearance of the light between the knuckle, 
or end of the finger, and the refpedive tubes ; but 
yet may be farther confirmed by obferving, that a 
polifhed glafs tube, when excited by fmooth oiled 
filk, if the hand be kept at leaft three inches from the 
top of the rubber, will at every ffroke appear to throw 
out a great number of diverging pencils of eledric 
fire, as in Fig. 3. Plate 33 } but not one was ever feen 
to accompany the rubbing of fulphur, fealing-wax, 
&c. nor was I ever able to make any fenfible altera- 
tion in the air of a room, merely by the fridion of 
thofe 
