Blowpipes may be considered as miniature blast-furnaces. They are
little instruments used to force--by means of air blown from the
mouth--the flame of a lamp or candle into a jet of flame so fierce
that the very highest heat can be produced by it. Various forms
of blowpipes are shown in the figures; the common blowpipe, used
by gas-fitters, tinmen, &c., is shown at _a_; better blowpipes
have generally some reservoir to contain the condensed breath and
so prevent it issuing into the jet; the bulb shown at _b_ is for
this purpose and also the conical part of _c_. Very good and cheap
blowpipes may be made by bending a piece of glass tube into the
form shown at _d_, adding a perforated cork and a small piece of
bent glass tube fixed as in the figure. The end of the small tube,
intended to produce the jet, should be held in the flame of a lamp
or gas till it is red hot and turned round all the while; in this way
the hole will gradually become smaller as the melted sides collapse,
forming a neat round hole about the size to admit a fine needle; with
this blowpipe a very great heat can be produced, and it can be easily
repaired. The oxy-hydrogen blowpipe is a contrivance for forcing a
jet of oxygen and hydrogen gases--mixed together in the proportions
in which they form water--through a small orifice and setting fire
to it; this produces the very highest heat. Almost any substance can
be fused by it, but the experiment should not be made unless with
a proper apparatus, as the flame will be sure to run down the tube
and explode the mixed gases with dangerous violence. A common flame
is merely a cone of vapour burning on the surface where it comes
into contact with the air, and therefore gives out but little heat,
but when air is forced into it, a small blue cone of solid flame is
projected, which gives off more heat than the hollow cone.