[ 45'5 ] 
The 1 1 ourmaltn we find, when uniformly aiffurb- 
ed on ail Tides, by changing the temperature of the 
air ; is not only electrified, but Thews two oppofite 
and contrary effeCts. That is, in pafiing from a great, 
to a lefs, degree of warmth, it is ele&rified in one 
manner ; and in pafiing from a lefs to a greater de- 
gree of warmth, it is electrified in another manner; 
which evidently (hews, that there is lome power , be- 
longing to the (lone, which is differently affeCted by 
fuch contraction and dilatation. The fame thing appears 
from other different effeCts it affords, when its two fides 
are equally warm. But the ‘ Tourmalin , affording no 
eleCtric appearance whatever, when the whole mafs 
is of the fame temper with the air, agrees with the 
obfervation that all bodies ceafe in that l late , to con- 
tract or dilate : and is a manifeft indication, that the 
fluid , which caules thefe eleCtirc appearances, is in fuch 
circumftances, in equilibria ; and muff ever remain 
fo, unlefs difturbed by violence. 
The importance of this laff obfervation befpeaks 
your attention, as it greatly tends to throw more light 
upon this curious fubjedt. 
From the experiment with the bent tube, menti- 
oned in the former part of this letter, it was proved, 
that there is a refljlance exerted at, or near, the furface 
of the Quickfilver where the light is accumulated. 
This refljlance , which I apprehend is effential to all 
bodies, merits a further illuftration ; becaufe the elec- 
tric phenomena in general greatly depend upon it. 
When a bladder is well blown up, and fecured 
properly, it will yeild or give way, and change its 
form, in that part againff which any given preffure is 
exci ted . 
