6(;o G L 
in fonie inilances product a compound poITening greater 
fufibility than belonged to any of the ingredients. 
The-earthy fubflances made ufe of in the coinpofition 
•of glafsare, quartzofe, fand, flint, gypfeous fpar, chalk, 
and the like, either finely, or in various proportions ; 
except, however, that the filiceous earth is nevtr to be 
difpenfed with. The faline additions confift either ot 
barilla or Ibda, common pot-afli, or vvood-afl;es, and 
amount to about one-third part of the whole. Thefe 
i'ubflanccs are purified and pounded with greater or lefs 
care, according to the purity or excellence of the glafs 
intended to be produced. If the materials were put into 
a velfel and expofed to fufion, they would froth and 
fwcll much, partly on account of the efcape-of moifture 
and other volatile matter, and partly by reafon of I'ome 
elaflic fluid which appears to be extricated at the time ' 
of the combination of the-balls and the flux : various 
impurities of an inflammable nature w'ould likewife re¬ 
main unconi'umed in the body of the glafs, and injure 
its colour. To prevent thefe inconveniences, the com- 
pofition is kept for the fpace of four and twenty hours 
in a furnace of a particular conftrudfion, at a degree of 
heat not fufflcient to produce complete fufion, and with 
accefs of air. It is repeatedly ftirred during this calci¬ 
nation, and becomes at length converted into an imper- 
fcft glafs called frit. This is put into the melting-pots, 
and filled by the Itrong heat of a reverberatory furnace. 
The fufed glafs has then the property of adhering to 
an iron rod or tube, by which means it is taken out, 
either to afcertain its ifate of perfection, or to blow it 
into fucli utenlils as may be required. The unvitrifiable 
falts, or fuch as do not enter into combination with the 
earth, fwim at tlie top of the veflel, forming the fub- 
thince called /iwiflfmcr, or glafs gall, which is fcummed off. 
The procefs of the glafs-blower is fimply this: 1 he 
fufed'glafs is taken out by dipping an iron tube into the 
pot; and the quantity fiiit dipped out is fuffered to cool 
•a little, and ferves as the receptacle for more glafs to be 
taken up at a fecond dip, and fo on, until the quantity 
for blowing is fufficient. The lump of glafs may be 
Ibftcned at pleafure, by holding it before the mouth of 
the furnace. The workman renders it hollow, and of a 
round or fpherical form, by blowing througli the tube. 
This fphere may be converted into a cone, a cylinder, 
or any otlier folid, whofe tranfverfe feClion is a circle, 
by rolling it on a flat plate of iron. It may be flretched 
or lengthened by fwinging the tube in the air, or giving 
it a vibratory motion like that of a pendulum. Tlie 
workmen evince great dexterity in heating the glafs in 
the various flages of the manipulation. They do this 
in fuch parts as they are defirous of extending ; and on 
other occafions they cool certain parts of their work, by 
fanning the air againfl it. The glafs, in the ignited Ilate 
it poll'efl'es after it comes out of the pots, is fo tough and 
flexible, tliat it may be cut with Ihears, bended with 
pincers, preifed into moulds, and wrought in a variety 
of methods dependent on thele properties, of which the 
artifts very dexteroully avail themfelves. Thus are all 
kinds of glafs veflels and figures made ; but flat or plate 
glafs is produced by running the ignited vitreous metal 
into tabular moulds. 
As far as obfervation has hitherto direCled us, it ap¬ 
pears to be a general rule, that the hardnefs, brittlenels, 
elaflicity, and other mechanical properties, of congealed 
bodies, are greatly affebfed by the degree of rapidity 
with which they cool, or afiTume the folid ftate. This, 
which no doubt is referrible to the property of cryftalli- 
zation, and its various modifications, is remarkably feen 
in fieei and other metals, and obtains in glafs. When a 
drop of fufed glafs is fuffered to fall into water, it is 
found to poiTels the remarkable property of flying into 
minute pieces, the inifant a fmall part of the tail is 
broken ofi. This, which is comaionly diftinguifhed by 
the name ot Prince Rupert's drop, is limilar to the philo- 
iophical phial, v/hich is a fmall velfel of thick glafs 
ASS. 
fuddenly cooled by expofure to the air. It pofTefTeSi 
the remarkable property of flying in pieces when the 
fmalleft piece of flint oriangular pebble is let fall into 
it. Many explanations have been offered to account 
Tor tlicfe and other fimilar phenomena, by referring to 
a fuppofed mechanifm or arrangement of the particles, 
or fudden confinement of the caloric, or matter of heat. 
The immediate caufe, however, appears to he derived 
from the fa6f, that the dimenfions of bodies fuddenly 
cooled, remain larger than if the refrigeration had been 
more gradual. Thus the fpecific gravity of fieel har¬ 
dened by fudden cooling in water is lefs, qnd ifs dinien. 
fions confequently greater, tlian that of the fame fieel 
gradually cooled. Hence it is probable that an effedt of 
the fame nature obtains in glafs ; fo tiiat the dimen¬ 
fions of the external and fuddenly-cooled furface remain 
larger tlian are fuited to the accurate envelopement of 
the interior part, which is more flowly cooled. In moft 
of the metals, the degree of flexibility they poffefs miifi 
be fuflicient to remedy this inaccuracy as it takes place'; 
But in glafs, which, though very elafiic and flexible, is 
likewife exceifively brittle, the adaptation of the parts, 
urged different ways by their difpolition to retain their 
refpective dimenfions, and likewife to remain in contadf 
by virtue of the cohefive attradtion, can be maintained 
only by an elafiic yielding of the whole, as far as may 
be, which will therefore remain in a ftate of tenfion. It 
is not, therefore, to be wondered at, that a folution of 
continuity of any part of the furface fiiould deftroy this 
equilibrium of elafticity ; and that the fudden action of 
all the parts at once, of fo brittle a material, fiiould de- 
firoy the continuity of the whole, infiead of producing 
an equilibrium of any other kind. 
Though the fadts relating to this difpofition of glafs 
too fuddenly cooled are numerous and interefting to the 
philofopher, yet they confiitute a ferious evil with re- 
fpedl to the ufes of this excellent material. The remedy 
of the glufs-maker confifts in annealing the I'everal arti¬ 
cles, which is done by placing them in a furnace over 
the furnace of fufion. The glalfes are firft put into the 
hotteft part of this furnace, and gradually removed to 
tlie cooler parts at regular intervals of time By this 
means the glafs cools very fiowly throughout, and is in 
confequence free from the flaws and defedts of glafs 
which has been too haftily cooled. See the article An¬ 
nealing, vol. i. p.733. 
It is difficult to fpeak with exadt precifion concerning 
the materials, proportion, and management necelTary to 
make the dift'erent kinds of glafs, fuch as the green glafs 
for bottles, the common green and crown glafs for win¬ 
dows, the white glafs for mirrors, the white flint glafs 
for decanters and jars, and the cryftal glafs iifed for the 
finer cut glafs, not to mention the denfe white glafs 
made exprefsly for optical infiruments. All thefe are 
made of a better quality at lome maiuifadlorres than at 
others; and it is probable that this fuperiority of pro¬ 
duce is rendered fecret by the natural difpofition for gain, 
which prevails among all men whofe pecuniary fuccefs 
depends on their monopolizing the eftedls of their own 
Ikill. The green glafs is made trom inferior materials; 
the bafis confifiing of a ferruginous fione or fand, and 
the alkali fuch as can be the moft cheaply obtained. 
The colour principally depends on the iron; and the 
glafs is harder, more durable, and lefs deftruTible by 
acids remaining in it for a long time, the lefs the quan¬ 
tity of alkali, and confequently the greater the heat. 
Chaptal, in his Elements of Chemiftry, mentions the 
I'liccefsful eftabliffiment of a manufaitory of opaque bot¬ 
tles of exceflive ftrength and lightnefs, compol’ed chiefly 
of bal'altes; but he relates that the eltabliffiment failed, 
chiefly on account of the quality of the bafaltes, which 
did not conftantly prove the fame, but, becoming in the 
latter ftages of the undertaking more calcareous, pro¬ 
duced an article of a perilhable nature, and fubjeCc to 
flaws. All the white glafl'es owe their ciearnels to the 
purity 
