[ 902 ] 
When the preceding experiments were firft: made, 
I was a little eth bar raffed with the uncertain appear- 
ances of a plus eledricity at one time, and a minus 
at another, in the fame experiment : but, by repeated 
trials and obfervations, I have found that a plus or 
minus eledricity may be produced at pleafure, by 
carefully attending to the following circumftances, 
viz, the form of the bodies, their ludden or gradual 
removal, and the degrees of eledrifying. 
I fihall now proceed to acquaint your Lordfhip 
with fome other circumftances of as nice a nature, 
where the flighted:, and almoft imperceptible dif- 
ferences in the pojition , or in the force of the frittion , 
of two bodies, produce in either of them, the plus 
eledricity at one time, and the minus at another. 
Such are the effeds of this fubtile and adive fluid, 
when the experiments are carefully made j and there- 
fore, they require the moft fcrupulous attention, to 
trace out the caufes which occaflon them. 
Sealing wax and filver were the bodies ufed in the 
firft two experiments j but many other fubftances feem 
to perform as well. The fealing wax was clean, and 
undifturbed by any fridion whatever, but that of the 
air furrounding it ; and had been fo for fome hours. 
The filver was fixed to a piece of the prepared wood, 
which was alfo preferved from fridion for the fame 
length of time. Then taking one of thefe fubftances 
in each hand, the filver being at that end of the wood 
the fartheft from the hand, I laid the fmootheft part 
of the filver upon the fealing wax, and moved it along 
the furface gently, once only , and with a very flight 
prejjure ; after which, the filver was eledrified plus, 
and the wax minus. 
On 
