olybdenic acid with some
borax, upon a platinum wire, about the sixteenth of an inch from the
point of the blue cone. In the pure oxidation flame, a clear yellowish
glass is formed; but as soon as the reduction flame reaches it, or the
point of the blue cone touches it, the color of the bead changes to a
brown, which, finally, after a little longer blowing, becomes quite
dark, and loses its transparency. The cause of this is, that the
molybdenic acid is very easily reduced to a lower degree of oxidation,
or to the oxide of molybdenum. The flame of oxidation will again
convert this oxide into the acid, and this conversion is a good test
of the progress of the student in the use of the blowpipe. In cases
where we have to separate a more oxidizable substance from a less one,
we use with success the blue cone, particularly if we wish to
determine whether a substance has the quality, when submitted to heat
in the blue cone, of coloring the external flame.

A good _reduction_ flame can be obtained by the use of a small orifice
at the point of the blowpipe. In order to produce such a flame, hold
the point of the blowpipe higher above the wick, while the nozzle must
not enter the flame so far as in the production of the oxidation
flame. The point of the blowpipe should only touch the flame, while
the current of air blown into it must be stronger than into the
oxidation flame. If we project a stream, in the manner mentioned, into
the flame, from the smaller side of the wick to the middle, we shall
perceive the flame changed to a long, narrow, luminous cone, _a b_,
Fig. 4, the end _a_ of which is enveloped by the same dimly visible
blueish colored portion of the flame _a, c_, which we perceive in the
original flame, with its point at _c_. The portion close above the
wick, presenting the dull appearance, is occasioned by the rising
gases which have not supplied to them enough oxygen to consume them
entirely. The hydrogen is consumed, while the carbon is separated in a
state of bright ignition, and forms the internal flame.

[Illustration: Fig. 4]

Directly above the wick, the combustion of the gases is least
complete, and forms there likewise, as is the case in the free flame,
a dark blue nucleus _d_.

If the oxide of a metal is brought into the luminous portion of the
flame produced as above, so that the flame envelopes the substance
perfectly, the access of air is prevented. The partially consumed
gases have now a strong af