turbid
and opalescent; this solution contains silica and potash combined
together in the proportion of 70 to 30.

Silicate of soda is made with 180 parts of sand, 100 parts carbonate of
soda (0.91), and is to be melted in the same manner as indicated
previously.

Soluble glass may also be prepared by the following method: A mixture of
sand with a solution of caustic potash or soda is introduced into an iron
boiler, under 5 or 6 atmospheres of pressure, and heated for a few hours.
The iron boiler contains an agitator, which is occasionally operated
during the melting. The liquid is allowed to cool until it reaches 212 deg.,
and is drawn out after it has been allowed to clear by settling; it is
then concentrated until it reaches a density of 1.25, or it may be
evaporated to dryness in an iron kettle. The metal is not affected by
alkaline liquors.

The glass is soluble in boiling water; cold water dissolves but little of
it. The solution is decomposed by all acids, even by carbonic acid.
Soluble glass is apparently coagulated by the addition of an alkaline
salt; mixed with powdered matters upon which alkalies have no effect, it
becomes sticky and agglutinative, a sort of mineral glue.

To apply soluble glass for the preservation of buildings and monuments of
porous materials, take a solution of silicate of potash of 35 deg. Baume,
dilute it with twice its weight of water, paint with a brush, or inject
with a pump; give several coats. Experience has shown that three coats
applied on three successive days are sufficient to preserve the materials
indefinitely, at a cost of about 15 cents per square yard. When applied
upon old materials, it is necessary to wash them thoroughly with water.
The degree of concentration of the solutions to be used varies with the
materials. For hard stones, such as sand and free stones, rock, etc., the
solution should mark 7 deg. to 9 deg. Baume; for soft stones with coarse grit, 5 deg.
to 7 deg.; for calcareous stones of soft texture, 6 deg. to 7 deg.. The last coating
should always be applied with a more dilute solution of 3 deg. to 4 deg. only.

Authorities are divided upon the successful results of the preservation
of stone by silicates. Some claim in the affirmative that the protection
is permanent, while others assert that with time and the humidity of the
atmosphere the beneficial effects gradually disappear. It might be worth
while to experiment upon some of the porous sandstones, whi