[ 9°4 ] 
Mr. Dufay, and the Abbe Nollet, have indeed 
obferved an inconffancy in the effects, when they 
rubbed fealing wax ; but for want of the cautions 
above-defcribed, in the choice and treatment of the 
bodies, they were not able to anfvver for the event of 
any experiment. The Abbe, in particular, who has 
taken remarkable pains to find out from whence this 
uncertainty arifes, acquaints us, in the thirteenth letter , 
with his difficulties, and how much it perplexed him 
to fee, within an hour, in the fame place, and with 
the fame inffruments, the effects changed, and be- 
come altogether contrary to thofe he had before ob- 
ferved. In another part of the fame letter, he fays : 
“ Je me fuis flatte plus d’une fois d’avoir faifi les 
“ circonffances qui faifoient tourner a coup fur le re- 
“ fultat de l’experience d’un cote ou de l’autre j mais 
“ la fuite m’a fait connoitre que je m etois trompe, 
“ ou qu’il manquoit encore quelque chofe a ma de- 
“ coverte.” 
I might add other quotations of the fame kind, 
but, as it would take up too much of your Lordfhip’s 
time, I fball beg leave to refer to the letter itfelf. 
From the experience now obtained, it may not be 
improper to obferve farther, that fealing wax of dif- 
ferent hardnefs, will occafion a difference in the ef- 
fects, the hardeff being always plus , and the fofteff 
minus ; which agrees very well with the rule laid 
down in the letter upon the Tourmalin, p. 33 1, 
et that where eledfric appearances are produced by 
“ the rubbing of any two polifhed bodies together, 
“ that body, whole fubflance is hardeff, and eledtric 
“ power ffrongeff, will be always plus, and the fofteff 
st and weakeff always minus'* Now we find that 
the 
