28 Prof. Fornes’s Experiments on the Electricity of Tourmaline 
charged with vitreous or resinous electricity ; and the repulsions 
occasioned by the presence of the body under experiment, are 
measured with sufficient exactness by the deviation of the needle 
reckoned on the divided circle of paper 7k. I may add, that the 
graduated circle H at the top of the glass tube is employed to 
measure any required torsion of the silk fibre produced by causing 
the cork F to revolve. By means of this simple apparatus, I 
obtained results of greater accuracy than my objects generally 
required *. 
Upon presenting to the electrometer, which is quite an insu- 
lator, an energetic crystal of tourmaline (which should not be too 
thin), heated to a considerable temperature, the gilt disk being 
charged with the same electricity as is acquired in cooling by 
the pole of the crystal presented, the following results appear :— 
At first the tourmaline exerts no influence whatever. But after 
its temperature has begun to fall, the repulsive power is gradual- 
ly developed, and the gilt disk slowly recedes as the increasing 
force appears ; the recession becomes very minute, and at length 
reaches a maximum at which the needle remains for a time sta- 
tionary. Soon, however, a diminution of repulsion takes place, 
the disk reapproaches its original point of rest, and, if left long 
enough, will return to the zero point precisely opposite the crys- 
tal, which then as at first produces no action whatever. It is 
unnecessary to point out how completely this verifies M. Bxc- 
QUEREL’s views, and demonstrates that, as soon as the tempera- 
ture completely ceases to change, not the minutest vestige of 
electricity remains, though the insulation throughout should 
be maintained as completely as possible. This I generally ac- 
* It is surprising that Coutoms’s instrument has not been more employed in 
this inquiry. BrcquEREL seems only to have used it once, and Dr Brewster had 
recourse to the laborious and unsatisfactory method of causing pyro-electric crystals 
to lift fragments of the Arundo Phragmites, which can give no comovarative re- 
sults. 
