HOCK CRYSTAL. 45 
kind of millstone. It is chiefly found in France ; but 
is so much esteemed by the English millers, that the 
Society of Arts, in London, for many successive years, 
offered a considerable reward for its discovery in Great 
Britain. At length a vein of burrstone was discover- 
ed in the Moel y Golfa hills, North Wales, by a 
Mr. Evans, who, in consequence received a premium 
from the Society. About the same time another 
vein was opened near Conway ; and the same Society, 
in 1800, gave a premium of 100/. to the widow and 
orphan children of the discoverer. Both these quarries 
were sufficiently convenient for water carnage ; yet the 
demand for the Cambrian burr did not answer the ex- 
pectation, and millstones of French production were 
still preferred to them. 
The mode of splitting these stones, as it is practised 
in some parts of France, is singular, and affords a proof 
of the extraordinary power of capillary attraction. The 
blocks are first cut into the form of cylinders, some- 
times many feet in height. To split these horizon- 
tally into millstones, circular indentations are made 
round them, at proper distances, according to the 
thickness that is to be given to the stones ; wedges of 
willow, that have been dried in an oven, are then 
driven into the indentations with a mallet. When these 
have been sunk to a proper depth, they are moistened 
with water ; and, after a few hours, the several stones 
that have been marked out are found to be perfectly 
separated. 
78. ROCK CRYSTAL is an extremely beautiful kind of 
quartz, sometimes perfectly transparent, and sometimes shaded 
with grey, yellow, green, brown, or red. It occurs in the 
form of crystals with six sides, each terminated by a six-sided 
prism. 
The name of this substance was considered by the 
ancients to signify ice, or water crystallized ; and they 
imagined that crystal was produced from a congelation 
of water. 
