INTRODUCTORY.If an abundant air blast is available at a fairly steady pressure,
the most convenient and useful form of furnace is the gas
furnace, Fig. 1, made of fire clay supported by a casing of
sheet iron, into which the flame from a large blast lamp is
admitted through a hole near the bottom. In the absence of an
air blast, gasolene blowpipes, similar to those commonly used by
plumbers, serve almost as well as the gas blast lamps. The use
of charcoal furnaces is inexpensive and to be recommended even
at the present day. Such a furnace is shown in Fig. 2, one-tenth
its natural size. It is made of sheet iron and is about 250 cm.
high and 280 cm. wide. Larger furnaces may be built of fire
brick and connected with the chimney of the building. The
fire in such a furnace can be started with some glowing charcoal
which has been heated before the blast lamp; the furnace is then
fed with layers of charcoal and coke and finally with coke alone.
Fig. 2. Fig. 3.For heating substances in glass or porcelain tubes, a flamespreader placed on an ordinary Bunsen burner is used when only
short lengths are to be heated; for longer stretches a row burner,
as shown in Fig. 3, is employed, and to retain the heat a cover of
asbestos board can be supported just above the tube by means of
wires. The three supports for the tube can be protected by
wrapping wet asbestos pulp about them, pressing it firmly intoposition and allowing it to dry. For higher temperatures over a
considerable length of tube a combustion furnace, such as is used