Cryjlalllmtlons obferved on Glafs. 535 
oblong and needle-like, and others globular, or nearly of 
the globular form. In this piece of glafs, many of the 
needle-like cryftals are .feen to unite round a common 
center; and although they have probably been prevented, 
by the too Hidden cooling of the glafs, from concreting- 
in a fufficient number to make complete globular cry- 
flals, yet they fufficiently fhew the manner in which 
thofe, which are complete, have been formed. All the 
cryftallizations, hitherto defcribed, were obferved in a 
dark, green window-glafs, made at Stourbridge, and called 
Broad-glafs. This glafs is compofed of fand, kelp, cal- 
careous earth, and lixiviated vegetable allies. 
Cryftallizations frequently occur alfo in the glafs of. 
which common bottles are made, the materials ufed in 
the compofition of which are nearly the fame as thofe 
above mentioned for broad-glafs, with the addition fome- 
times of the fcoria of iron furnaces. Of this kind is the 
fpecimen marked N° 5. in which the cry ifals are not in- 
veloped in a medium of tranfparent uncryflallized glafs, 
for the whole piece is an opaque, cryftallized fubftance ; 
but they are prominent from the furface of the mafs. 
The form of the cryftals is that of the blade of a two- 
edged fword, whofe point is truncated. In no other 
glafs have I feen fuch perfect cryftals as in thofe two kinds, 
above mentioned, broad and bottle-glafs, which being- 
more fluid and lefs tenacious, when melted, than any 
other, the minute particles of which cryftals confift, 
more eafily concrete, and apply themfelves to each other, 
with lefs reliftance from the medium. Perhaps alfo the 
