If the flame of a candle or lamp be closely examined, it will be seen to
consist of four parts--(a) a deep blue ring at the base, (b) a dark cone
in the centre, (c) a luminous portion round this, and (d) an exterior
pale blue envelope (see FLAME). In blowpipe work only two of these four
parts are made use of, viz. the pale envelope, for oxidation, and the
luminous portion, for reduction. To obtain a good _oxidizing flame_, the
blowpipe is held with its nozzle inserted in the edge of the flame close
over the level of the wick, and blown into gently and evenly. A conical
jet is thus produced, consisting of an inner cone, with an outer one
commencing near its apex--the former, corresponding to (a) in the free
flame, blue and well defined; the latter corresponding to (d), pale blue
and vague. The heat is greatest just beyond the point of the inner cone,
combustion being there most complete. Oxidation is better effected (if a
very high temperature be not required) the farther the substance is from
the apex of the inner cone, for the air has thus freer access. To obtain
a good _reducing flame_ (in which the combustible matter, very hot, but
not yet burned, is disposed to take oxygen from any compound containing
it), the nozzle, with smaller orifice, should just touch the flame at a
point higher above the wick, and a somewhat weaker current of air should
be blown. The flame then appears as a long, narrow, luminous cone, the
end being enveloped by a dimly visible portion of flame corresponding to
that which surrounds the free flame, while there is also a dark nucleus
about the wick. The substance to be reduced is brought into the luminous
portion, where the reducing power is strongest.