te of fusion until all gases
formed in the reaction have escaped.

_Molding and blowing of glass._ The way in which the melted mixture is
handled in the glass factory depends upon the character of the article
to be made. Many articles, such as bottles, are made by blowing the
plastic glass into hollow molds of the desired shape. The mold is first
opened, as shown in Fig. 74. A lump of plastic glass A on the hollow
rod B is lowered into the mold, which is then closed by the handles
C. By blowing into the tube the glass is blown into the shape of the
mold. The mold is then opened and the bottle lifted out. The neck of the
bottle must be cut off at the proper place and the sharp edges rounded
off in a flame.

Other objects, such as lamp chimneys, are made by getting a lump of
plastic glass on the end of a hollow iron rod and blowing it into the
desired shape without the help of a mold, great skill being required in
the manipulation of the glass. Window glass is made by blowing large
hollow cylinders about 6 ft. long and 1-1/2 ft. in diameter. These are
cut longitudinally, and are then placed in an oven and heated until they
soften, when they are flattened out into plates (Fig. 75). Plate glass
is cast into flat slabs, which are then ground and polished to perfectly
plane surfaces.

_Varieties of glass._ The ingredients mentioned above make a soft,
easily fusible glass. If potassium carbonate is substituted for the
sodium carbonate, the glass is much harder and less easily fused;
increasing the amount of sand has somewhat the same effect. Potassium
glass is largely used in making chemical glassware, since it resists the
action of reagents better than the softer sodium glass. If lead oxide is
substituted for the whole or a part of the lime, the glass is very soft,
but has a high index of refraction and is valuable for making optical
instruments and artificial jewels.

[Illustration: Fig. 75]

_Coloring of glass._ Various substances fused along with the glass
mixture give characteristic colors. The amber color of common bottles is
due to iron compounds in the glass; in other cases iron colors the glass
green. Cobalt compounds color it deep blue; those of manganese give it
an amethyst tint and uranium compounds impart a peculiar yellowish green
color. Since iron is nearly always present in the ingredients, glass is
usually slightly yellow. This color can be removed by adding the proper
amount of manganese dioxide, for th