Obfervations in Ele&ricity \ 1 1 g 
electrified (either positively or negatively, according to 
the nature of the fubftance and form of its furface, and 
the quality and furface of the rubber) Sufficiently to Se- 
parate the balls of the electrometer, So as to determine 
their electricity in a very Satisfactory manner. This, I 
think, fully confirms an opinion I have long entertained 
and mentioned in a former paper, viz. that the Slightest 
friction of bodies of every kind, in every fituation, may 
difturb the electric matter contained in them, though the 
effect be imperceptible to us, having no electrometer nice 
enough to difcover it. Here, therefore, is a boundlefs 
field for future enquiry; and, to affift thofe who may be 
inclined to profecute it, I Shall mention a few general ob- 
servations and precautions, which I have found exceed- 
ingly ufeful in the courfe of my own experiments, iff,. 
The air Should be dry, and the apparatus clean and warm, 
adly, The fubftances to be tried Should be perfectly clean. 
3dly, When the rubber hath been ufed once or twice, it 
Should be held near the fire or the flame of a candle, 
not only to prevent its acquiring moifture, but to take off 
the electricity left in it by one Substance, before another 
be examined ; for it Should always be remembered, that 
whenever a fubftance is made eleCtrical by friCtion, the 
rubber acquires the contrary electricity, and this electri- 
city, if it be not carefully taken off as is above directed, 
will 
