[« 4 ] 
I have filled feveral fmall glafs tubes with the dry- 
powders of calcined metals, viz. cerufs, lead afhes, 
minium, calx of antimony, & c. Into each end of every 
tube I put a piece of iron wire, which communicated 
with the calx, and fattened them with wax : fo that 
the eledtric fluid, not being able to efcape by means 
of the glafs, mutt either pafs thro’ the calx, or not 
at all. Upon hanging one of the wires, bent for the 
purpofe, to the eledtrified bar, and holding the other 
in my hand, I obferved that no eledtric matter did 
pafs the calx, the fnaps ifiuing all the while from 
the bar, or from that wire which was in contadb 
with the bar *. 
Animal and vegetable folids alfo, when reduced to 
afhes, and interpofed in the fame manner between 
two pieces of wire, do, I find, as effedtually inter- 
cept the eledtric ttream, as the metallic calces. 
From thefe experiments you fee, that animal, ve- 
getable, and metallic bodies, tho’ fuch known con- 
dudtors of the eledtric fluid while in their intire 
ttate, are eafily changed into refifters or non-con- 
dudtors of it. 
I was led to attempt this change from its having 
been obferved, that dry mould would not condudt 
the eledtric fluid : and from thence I lufpedted, that 
one clafs of the non-condudtors mutt owe its property 
to an eledtrical virtue that would be found to refide 
in the calx, or earth of the chymifts, after it is di- 
vetted of the undtuous inflammable matter, which 
* Since I wrote this letter, I have been informed, that part of 
this firft experiment, relating to metallic calces, has been made 
before by Dr. Watfon. See the note Phil. Tranf. vol. xlv. p. 107. 
confli- 
