Mr. keir on CryJlallizationSy &c. 531 
In many fubftances, when broken, the parts appear to 
have fome determinate figure. This determination of* 
figure, or grain, as it is called, is obvious in bifmuth, re- 
gulus of antimony, zinc, and all other metallic bodies, 
which may be broken without extenfion of parts ; and 
although the duality of gold, filver, lead, and tin, pre- 
vents the appearance of the peculiar grains, when pieces 
of thefe metals are broken, yet we have reafon to believe, 
that, by expofing them to proper circumftances, they alfo 
would fhew a difpofition to this fpecies of cryftallization, 
as it may be called, by a further extenfion of that term ; 
for Mr. homberg has obferved, that when lead is broken 
while hot, in which ftate it is not dudtile, a granulated 
texture appears. Perhaps all homogeneous bodies, in 
their tranfition from a fluid to a folid ftate, would, if this 
tranfition were not effedted too haftily, concrete into cry- 
ftals, or bodies fimularly figured. In fiances of fuch cry- 
flallization have occurred to me in glafs, which had palled 
very flowly from a fluid to a folid ftate ; and the form, 
regularity, and fize of thefe vitreous cryftals have varied 
according to the cncumilances with w T hich their concre- 
tion had been accompanied. I fend along with this 
paper a few fpecimens of this cryftallized glafs, together 
with a drawing of fome of the moll remarkable cryftals. 
The pieces of glafs, marked N° 1 . were taken from the 
bottom of a large pot, which had flood in a glafs-houfe 
furnace at the time the fire was allov r ed gradually to ex— 
tinguifh. In this cafe, the mafs of heated matter was lb 
great that, without the addition of fuel, the heat con- 
Vol. LXVI. 4 A tinued 
