to make a solution marking 33° to 35° Baume. Distilled or rain water
should be used, as the calcareous salts contained in ordinary water would
produce insoluble salts of lime, which would render the solution 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°,
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° 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