e by the
burning envelope surrounding it. On reaching the supporter of combustion
on the outside it is far above its kindling temperature, and combustion
follows with the evolution of much heat. The region of combustion just
outside the inner cone is therefore the hottest part of the flame.

[Illustration: Fig. 63]

~Oxidizing and reducing flames.~ Since the tip of the outside cone
consists of very hot products of combustion mixed with oxygen from the
air, a substance capable of oxidation placed in this part of the flame
becomes very hot and is easily oxidized. The oxygen with which it
combines comes, of course, from the atmosphere, and not from the
products of combustion. This outer tip of the flame is called the
_oxidizing flame_.

At the tip of the inner cone the conditions are quite different. This
region consists of a highly heated combustible gas, which has not yet
reached a supply of oxygen.

If a substance rich in oxygen, such as a metallic oxide, is placed in
this region of the flame, the heated gases combine with its oxygen and
the substance is reduced. This part of the flame is called the _reducing
flame_. These flames are used in testing certain substances, especially
minerals. For this purpose they are produced by blowing into a small
luminous Bunsen flame from one side through a blowpipe. This is a tube
of the shape shown in Fig. 64. The flame is directed in any desired way
and has the oxidizing and reducing regions very clearly marked (Fig.
65). It is non-luminous from the same causes which render the open
Bunsen burner flame non-luminous, the gases from the lungs serving to
furnish oxygen and to dilute the combustible gas.

[Illustration: Fig. 64]

[Illustration: Fig. 65]

~Luminosity of flames.~ The luminosity of flames is due to a number of
distinct causes, and may therefore be increased or diminished in several
ways.

1. _Presence of solid matter._ The most obvious of these causes is the
presence in the flame of incandescent solid matter. Thus chalk dust
sifted into a non-luminous flame renders it luminous. When hydrocarbons
form a part of the combustible gas, as they do in nearly all
illuminating gases and oils, some carbon is usually set free in the
process of combustion. This is made very hot by the flame and becomes
incandescent, giving out light. In a well-regulated flame it is
afterward burned up, but when the supply of oxygen is insufficient it
escapes from the flame as lampblack or