[ 87 ] 
in metallic fubdances upon mere pulverization, yet 
it feems to follow in moft other hard bodies. 
Having dried a piece of Portland done, I found 
it conducted perfectly well ; but upon powdering, 
and fealing it up in one of the tubes with the wire 
ends, as above, it became a perfedt redder, or non- 
conductor, like the metallic calces. 
I have tried the fame experiment on a variety of 
other bodies, particularly gum arabic and allum j 
and have reafon to believe it will fucceed in all 
bodies that can be pulverized in the mortar. 
Thefe lad experiments feem to confirm Sir Ifaac 
Newtons dodtrine of a medium furrounding all bo- 
dies, which you have applied to the folution of eledtric 
phenomena, and are very analogous to the experi- 
ments you made with a chain , in order to fhew that 
the reddance to the paffage of the eledlric duid may 
be increafed by increadng the number of furfaces. 
Another very extraordinary means of making this 
change in bodies, which abound in calx or earth, 
is by dre : not by the intenfe one that calcines, but 
by a moderate heat j their mod perfedt reddance, or 
non- conducting property, being when their heat is 
jud tolerable to our hands. 
I have fome of the dime Portland done, wrought 
into plates nearly as thin as window-glafs, which I 
heat to a proper degree, and then coat on both ddes 
with metal, in order to make the Leyden experi- 
ment. When the done is hot enough to dnge paper, 
it condudts as perfectly as when cold ; but on cooling 
a little, it begins not to condudt, and affords fmall 
diocks, which gradually increafe in drength for about 
ten minutes j at which time it is about its mod per- 
fedt 
