[ 8 6 ] 
and deflroy the effects of each other, and the body 
in which the pofitive and negative ones equally pre- 
vail, become neutral, or non-eledtric ? 
I have not fcrupled to rank thofe known pofitive 
eledtrics, glafs and tranfparent flones, under that clals 
of bodies which confifts of calx or earth ; becaufe 
all vitrifications mud proceed from previous calcina- 
tions, and all calces may be vitrified in the focus of 
large burning- glaffes. The tranfparent ftones alfo 
confift of little more than pure earth, free of the 
leafb mixture of oil, if we may judge of others by 
the chymical refolution of cryftal. 
There is another procefs, natural and without 
fire, which is fuppofed to dedroy the fulphureous 
fubftance of metals, viz. when they are corroded, 
and moulder in the open air. Accordingly, with the 
fame apparatus in which I tried the calcinations by 
fire, I examined the common rufl of iron, and flake- 
white, which is the ruft of lead, and find them 
equally converted into non-condudtors in the open 
air. 
That this change, in metals particularly, is not 
owing to, or promoted by, the circumdance of mere 
pulverization, is evident, not only becaufe the above- 
mentioned calces are equally ftrong eledtrics when 
formed into hard maffes with a thin pafte of flour 
and water, and afterwards dried, but mod; clearly 
becaufe the fined; filings or powders of metals con- 
dudt as readily as the intire fubftances do. I have 
glafs tubes armed as above, and filled with the pre- 
parations called powder of tin, &c. which condudt 
as well as a wire when it is not difcontinued. 
But notwithstanding this change will not fucceed 
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