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° to 9° Baume; for soft stones with coarse grit, 5°
to 7°; for calcareous stones of soft texture, 6° to 7°. The last coating
should always be applied with a more dilute solution of 3° to 4° 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, which, under the
extreme influence of our climate, rapidly deteriorate; such, for
instance, as the Connecticut sandstone, so popular at one time as a
building material, but which is now generally discarded, owing to its
tendency to crumble to pieces when exposed to the weather even for a few
years.

Soluble glass has also been used in Germany to a great extent for mural
painting, known as stereochromy. The process consists in first laying a
ground with a lime water; when this is thoroughly dry, it is soaked with
a solution of silicate of soda. When this has completely solidified, the
upper coating is applied to the thickness of about one-sixteenth of an
inch, and should be put on very evenly. It is then rubbed with fine
sandstone to roughen the surface. When thoroughly dry, the colors are
applied with water; the wall is also frequently sprinkled with water. The
colors are now set by using a mixture of silicate of potash completely
saturated with silica, with a basic silicate of soda (a flint liquor with