[ 9°3 ] 
On repeating the experiment, with equal care, and 
in the fame manner, except that the Jmooth Jide of 
the filver was a little inclined, fo that the edge thereof 
preffed againft the wax ; the filver, after moving it 
as before, was electrified minus , and the wax plus j 
which is contrary to what was obferved in the laft 
experiment. 
Thefe oppofite effeCts, occafioned by the different 
applications of the flat, or edge of the filver, feem 
to arife from an alteration made in the furface of 
the wax, by deflroying the polijh in one cafe, and 
not in the other : and, in this refpeCt, refembles the 
polifhed and rough glafs mentioned in the letter upon 
the Tourmalin. See p. 32S. of this Volume, Part I. 
Experiment 24. 
Upon making ufe of prepared wood, inftead of 
wax, and employing different degrees of preffure in 
the friction, with the fame edge of the filver, I pro- 
duced the like appearances ; the leaft preffure caufing 
a plus, and the greatefl preffure a minus appearance 
in the filver. 
A flat piece of fleel, well polifhed, and the edges 
rounded off, afforded the fame appearances, by only 
applying the flat furface to the wood ; but it required 
more pnefing to produce the minus effeCt in this cafe, 
than it did in the former, where the edge was concerned . 
Whether the reafon offered above for explaining 
thefe laft curious appearances be true or not, I do 
not venture to affirm, for want of farther experi- 
ments ; but thus much may be fafely advanced, that 
we have learned to produce at pleafure the plus or 
minus ekaricity from the fame bodies, by attending 
to the manner of their application and friction, 
Mr. 
