[ 35 6 ] 
There are feveral other earthy fubffances, I find, 
whofe electricity is deftroyed by very different degrees 
of heat. 
From confidering, that the degree of heat, at 
which the ifland cryftal, firft mentioned in this letter, 
is in its moff perfect ele£tric ffate, is lefs than the 
ufual heat of the air; and that a fmall increafe of that 
heat renders it non-eleCtric ; I do not think it im- 
probable, that many fubftances, which are not known 
to be eleCtric, may prove fo, if expofed to a greater 
degree of cold than they have hitherto been exa- 
mined in. 
I am, 
S I R, 
Your moff humble fervant, 
E. Delaval. 
V 
LVI. An 
