C 45 7 ] 
EXPERIMENT X. 
Upon a very nice examination, and during Tome fa- 
vourable opportunities, I have obferved the Tourmalin 
to be feebly electrified r , when the Therm, varied , up or 
down only one degree. 
The fmallnefs of the force here required to caufe 
thofe manifeft effects, and even them by natural means 
only, is a new difeovery, and, perhaps, deferves the 
attention of philofophers. 
In my firft and fecond letters upon the Tourmalin , 
there are experiments that give us affurances of a 
flux and reflux of the eledric fluid, or aether > at different 
times, even without artificial means to occafion it. And 
I did not fcruple to advance that dodrine, as appears 
from a paflage in the opticks which I quoted in the 
fecond letter, fomewhat favouring the fame opinion. 
This you fee has happened to be a right conjedure ; for 
thefe laft experiments, are I apprehend, fo clear and 
fatisfadory, that there is no room left for a doubt about 
it. And I do hope they will lead to fome ufefui 
difeoveries. For thefe forces, however fmall they may 
appear, are probably fufficient to anfwer very great 
purpofes in nature. 
Upon confidering the effeds which heat and cold 
occafion in the Tourmalin , it may not be improper 
here to obferve, that all bodies we are acquainted with, 
are dilated by heat, and contracted by cold : and when 
they acquire the fame temper with the air, whether 
it be hot or cold, the fame flate of dilatation, or con- 
tradion, continues unaltered. 
The 
