86 
from the mine called Stenna-Gwyn in the parish of 
St. Stephen near Branwell in Cornwall, and that the 
crystals, besides Water and Alumine, as the principal in- 
gredients, contain Lime and Silica in small proportions. 
One of these latter ingredients (or perhaps both of them) 
is probably united to Fluoric Aeid. This acid, as well as 
the Lime and Silica, seem to exist in a somewhat larger 
proportion in this fossil than in the common Hydrargillite. 
Some of the crystals were put into a platinum crucible, and 
Sulphuric Acid dropped upon them. On exposing the 
vessel to a moderate sand-heat, the vapour of Fluoric Acid* 
was extricated ; a piece of glass was put on the crucible, 
and a saline crust was deposited on it, which consisted of 
Fluoric Acid ana Silica: as the surface of the glass was not 
corroded, it is inferred that the Silica was detached from 
the fossil. The Sulphuric Acid effected a solution of the 
whole, except a small portion of Silica. On the evapora- 
tion of the fluid, some Sulphate of Lime was separated, 
as well as a little Silica; on adding some Potash to the 
remaining fluid, crystals of Alum were from time to 
time produced, after repeated evaporations, to the last 
drop. 
The angles of the mutual bases of the pyramids, were 
found to be about 102° and78°. The faces of the pyramids 
were near the base about 78° and 65°, making the summit 
37°. The depih of the truncation, which is a primitive 
face, varied. The nucleus is a cube. 
* Mr. Davy has some time since determined that a small portion of Fluo+ 
ric Acid, ina peculiar state, exists in the Devonshire Hydrargillite. 
