C 44$ ] 
time after, the fame Tourmalin , without being heated 
afrefh, or didurbed by any other caufe but that of 
the air furrounding, returned to its natural date again; 
that is, the convex fide changed to plus , and the plain 
Jide remained minus. 
One of the Tourmalins that were employed in thefe 
experiments, ( marked b ) belongs to Dr. Heberden , and 
is the fame I formerly made ufe of: but the larged of them 
was put into my poffeffion by the favour of Dr. Morton 
Secretary to the Royal-Society, of London , who re- 
ceived it, with feverai others, from Mr. Loton F. R. S. 
late governor of the Illand of Cylon. This done 
weighed about one hundred and eighty grains: but 
the form thereof not being the mod favourable for 
experiments, I had permifiion to fafhion it into fuch 
fnape as would anfwer the purpofe bed. For this end, 
alterations were made therein ; and at, one time, the 
eledtric poles (if they may be fo called) were pretty 
near in the longed direction of the done, and at 
another time, in the fhorted. Thefe trials terminated 
in the form and fize, nearly, as reprefented by the 
letter fig. 2 : and the direction of the poles is now 
in the fhorted line that can be drawn between any of 
the oppolite fides. 
I have to obferve, that when this Tourmalin is held 
between the eye and the light, and viewed in the 
diredfion though which the eledtrie fluid is found to 
pafs, irappears ot a darker colour confiderably, than 
when it is viewed at right angles to the former direc- 
tion. This appearance obtains in many other Tour- 
malins , efpecially if they happen to be as conveniently 
fhaped. 
In 
