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.

For the purpose of acquiring practice, the student may fuse the oxide