GLASS. 
mixed with the fixed alkalies, and exposed 
to a strong heat, readily enters into fusion. 
In this state the mixture may be moulded 
into any shape, and if suddenly cooled be- 
low the temperature at which it assumes 
the solid state, it retains the transparency, 
and those peculiar properties that belong 
to the substance called glass. Metallic 
oxides are sometimes added, as well to as- 
sist in thb fusion, as to communicate cer- 
tain colours to the mass. If the melted glass 
he suffered to cool very slowly, the diffe- 
rent tendency of the constituent part to 
assume solid forms at certain temperatures, 
will cause them to separate successively in 
crystals, as salts held in solution in water 
assume the form of crystals as the liquid is 
slowly evaporated. But if the glass he 
suddenly cooled down to the point of con- 
gelation, the constituents have not time, to 
separate in succession, and the glass remain!: 
the same homogeneous compound as while; 
in a state of fusion. Hence it should seem 
that the vitreous quality depends entirely 
1. upon the fusibility of the mixture ; and, 
2. on the suddenness with which it is cooled 
down to the point of congelation. It was 
discovered by Sir James Hall, that glass 
always loses its vitreous state, and assumes 
that of a stone, if more than a minute or 
two elapses while it is cooling down from 
•complete fusion to the point at which it 
Congeals. 
There are several kinds of glass adapted 
to different uses. The best and most beau- 
tiful are the flint and the plate glass. These 
when well made, are perfectly transparent 
and colourless, heavy and brilliant. They 
are composed of fixed alkali, pure siliceous 
sand, calcined flints, and litharge in diffe- 
rent proportions. The flint glass contain a 
large quantity of oxide of lead, which by 
certain processes is easily separated. The 
plate glass is poured in the melted state 
upon a table covered with copper. The 
plate is cast half.an inch thick, or more, and 
is ground down to a proper degree of thin- 
ness, and then polished. 
Crown-glass, that used for windows, is 
made without lead, chiefly of fixed alkali 
fused with siiliceous sand, to which is added 
some black oxide of manganese, which is 
apt to give the glass a tinge of purple. 
Bvttle-gla&s is the coarsest and cheapest 
kind : into this little or no fixed alkali en- 
ters the composition. It consists of an al- 
kaline earth combined with alumina and 
silica. In thus country it is composed of 
sand and the refuse of the soap-boiler, which 
consists of the lime employed in rendering 
his alkali caustic, and of the earthy matters 
with which the alkali was contaminated. 
The most fusible is flint-glass, and the least 
fusible is bottle-glass. 
Flint-glass melts at the temperature of 
10° Wedgewood ; crown-glass at 30°; and 
bottle-glass at 47°. The specific gravity va- 
ries between 2.48 and 3.33. 
Good glas3 is perfectly transparent, and 
when cold very brittle, but at a red heat 
it is one of the most ductile bodies known, 
and may be drawn into threads so very de- 
licate, as to become almost invisible to the 
human eye. It is extremely elastic, and 
one of the most sonorous of bodies. See 
Harmonica. 
There are but few chemical agents which 
have any action upon it. Mr. Davy in one 
of his lectures delivered in the course of the 
present month, (May, 1808,) exhibited a 
method of decomposing it by means of the 
Voltaic battery : he, however, first reduced 
it to powder. Fluoric acid, as w r e have 
seen, has a great power oter it, and dis- 
solves it very quickly (see Fluoric Acid) ; 
so also have the fixed alkalies when assisted 
by heat. The continued action of hot 
water is said to be capable of decomposing 
glass, which it is thought will fully explain 
how the siliceous earth was obtained by 
Boyle and others, when they subjected 
water to very tedious distillations in glass 
vessels. It has also been supposed, that 
the deflagration of the oxygen and hydrogen 
gases, in the formation of water, has de- 
composed the glass, which will account for 
an acid as part of the result. 
In making glass,' the materials are com- 
pletely fused together, and in this state the 
hot mixture is called frit. The frit is in- 
troduced into large pots made of prepared 
clay, and exposed to a heat sufficient to 
melt it completely. When the fusion has 
continued the proper time, the furnace is 
allowed to cool a little. In this state the 
glass is exceedingly ductile, and will assume 
any shape according to the fancy of the 
workman. The vessels thus formed, must 
not be permitted to pool too quickly, hence 
they are put into a hot furnace, in order 
that the heat may pass off very gradually : 
this is called annealing. 
Glass is often tinged of various colours, 
which is performed by mixing with it, while 
in fusion, some one of the metallic oxides. 
Thus blue glass is formed by the oxide of 
cobalt ; green by the oxide of iron, or cop- 
per ; violet by the oxide of manganese} 
