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 affinity for oxygen, under the influence of the intense heat of that part of the flame.  The substance is thus deprived of a part, or the whole, of its oxygen, and becomes reduced according to the strength of the affinity which the substance itself has for oxygen.  If the reduction of a substance is undertaken on platinum, by fusion with a flux, and if the oxide is difficult to reduce, the reduction will be completely effected only in the luminous part of the flame.  But if a substance be reduced on charcoal, the reduction will take place in the blue part of the flame, as long as the access of air is cut off; but it is the luminous part of the flame which really possesses the greatest reducing power.

The following should be observed in order to procure a good reduction flame: 

    The wick should not be too long, that it may make a smoke, nor
    too short, otherwise the flame will be too small to produce a
    heat strong enough for reduction.

    The wick must be free from all loose threads, and from
    charcoal.

    The blast should be continued for a considerable time without
    intermission, otherwise reduction cannot be effected.