346 
ON THE USES OF SOLUBLE GLASS. 
When the liquor becomes too thick, before 
the whole of the glass is dissolved, boiling 
water must be added. 
When the solution has acquired the consis- 
tence of syrup, and a density of 1.24 to 1 .25, 
it is sufficiently concentrated and fit for use. 
It is then permitted to rest, in order that the 
insoluble parts may be deposited ; while it is 
cooling, a coriaceous pellicle forms upon the 
surface, which after a time disappears of it- 
self or may be redissolved by depressing it in 
the liquor. This pellicle begins to appear 
during the ebullition, as theliquor approaches 
a state of concentration, and may even 
serve to indicate this state. 
When the crude glass is of a proper com- 
position, it contains but a few saline impuri- 
ties, and no sulphuret of potassium ; it may be 
treated in the way we have described. But 
if it contain any notable proportion of these 
substances, they must be separated before it is 
dissolved ; this separation may be effected in 
the following manner: — The powdered glass, 
is exposed to the action of the air for three or 
four weeks, during which time it must be fre- 
quently stirred ; and if it run into lumps, 
which will happen in moist weather, they 
must be broken up. I’he glass as we have 
stated attracts moisture from the air, and the 
foreign substances either separate or effloresce. 
It then becomes easy to remove them from the 
glass. It is sprinkled with water, and fre- 
quently stirred. At the end of three hours 
the liquor is removed, it will then contain a 
part of all the saline impurities, and a little of 
the silicate of potassa ; the powder is again to 
be washed with fresh water. Soluble glass 
thus treated, readily dissolves in boiling water, 
and the solution leaves nothing to be desired. 
As soluble glassis employed in the liquid 
form alone, it is kept in this state for use. To 
preserve it, no particular care is necessary, as 
even after a long space of time it undergoes 
no perceptible change, if the solution have 
been properly prepared. The only precau- 
tion is not to allow air too free an access to 
it. 
A similar product may be obtained by 
using a carbonate of soda instead of one of 
potassa. In this case, two parts of the soda 
of the shops is required for one of silica. This 
glass has the same properties as the other, but 
is more valuable in its uses. I'he solutions of 
these two kinds of glass may be mixed in any 
proportion whatever, and this mixture is more 
serviceable in some cases, than either of them 
separately. 
Properties. — Soluble glass forms a viscid 
solution, which when concentrated becomes 
turpid and opalescent; it has an alkaline 
taste and reaction. The solution mixes in all 
proportions with water. When the density 
of the solution is 1.25, it contains nearly 28 
per cent, of glass ; if the concentration be 
carried beyond this point, it becomes so viscid 
that it may be drawn out in threads like molten 
glass. Finally the liquor passes to the state 
of a vitreous mass, whose fracture is conchoi- 
dal ; it then resembles common glass, except 
In hardness. When the solution is applied 
to other bodies, it dries rapidly at common 
temperatures, and forms a coat like a varnish. 
Soluble glass, when dried, does not undergo 
any perceptible change when exposed to the 
air, and attracts from it neither moisture nor 
carbonic acid; neither has the carbonic acid 
of the atmosphere any well marked action ou 
the concentrated solution ; but when a cur- 
rent of carbonic acid is passed through the so- 
lution, the glass is decomposed, and hydrate 
of silica deposited. But a weak solution be- 
comes turbid on exposure to the air, and is after 
a time decomposed wholly. When the glass 
is impure, an efflorescence is formed after a 
while, which may be produced either by the j 
carbonate and hyposulphate. of potassa, or by 
chloride of potassium. 
Soluble glass dissolves gradually without 
residuum in boiling water ; but in cold water 
the solution is so slow as to have led to a belief 
that it does not dissolve at all. It however 
never becomes entirely insoluble, except when j 
it contains a much larger proportion of silica, or | 
when it is mixed with other bodies, such as the ' 
earths, metallic oxides, &c., with which double ' 
or triple salts are formed, as is the case in the 
common glasses. j 
Soluble glass which has been exposed to the 
air, and is afterwards submitted to the action of ? 
heat, swells and cracks at first, and melts with ! 
difficulty. It then loses about 12 per cent, of | 
its weight. It therefore contains, even when ’ 
solid, a considerable quantity of water, which j 
it does not lose when simply dried by exposure 
to the air. 
Alcohol precipitates it unaltered from its 
solution in water. When the solution is 
concentrated, but little alcohol is requiied for 
precipitation, and it need not be highly recti- 
fied. Pure soluble glass may therefore he easily 
obtained from an impure solution by the use 
of alcohol. The alcohol being added, the gelatin- 
ous precipitate is permitted to settle; the super- | 
nalantliquor is decanted, the precipitate collect- | 
ed, rapidly stirred after theaddilion of alittle cold 
water, and subjected to pressure . In truth, how- 
ever, this process is attended with some loss, for 
even cold water will rapidly dissolve the precipi- 
tated glass, in consequence ofitsminutedivision. 
The acids decompose the solution of glass. 
They also act upon it when solid, separating 
the silica in the form of powder. 
Uses.— The properties of soluble glass fit it 
for numerous and varied applications. It has 
been used in the theatre of Alunich as a means 
of safety from lire. | 
All sorts of vegetable matter, wood, cotton, 
hemp, linen, paper &c. are, as is well known, 
combustible ; but in order that they shall burn, 
two conditions are requisite, an elevated tem- 
perature, and free contact of air, to furnish the ' 
oxygen necessary for their transformation into 
water and carbonic acid. When once set on 
fire, their own combustion develops the heat 
necessary to keep up the chemical action, pro- 
vided they be in contact with air. If deprived 
of such contact, and made red hot, they will, it 
is true, yield inflammable volatile products, hut 
the carbon which is left will not burn, as it is 
deprived of air, and thus the combustion will 
slop of itself. Such is the part which all the 
