By inspecting the flame of a candle burning under usual circumstances, we perceive at the bottom of the flame a portion which is of a light blue color (a b), Fig. 2, which gradually diminishes in size as it recedes from the wick, and disappears when it reaches the perpendicular side of the flame.  In the midst of the flame there is a dark nucleus with a conical form (c).  This is enveloped by the illuminating portion of the flame (d).  At the exterior edge of the part d we perceive a thin, scarcely visible veil, a, e, e, which is broader near the apex of the flame.  The action of the burning candle may be thus explained.  The radiant heat from the flame melts the tallow or wax, which then passes up into the texture of the wick by capillary attraction until it reaches the glowing wick, where the heat decomposes the combustible matter into carbonated hydrogen (C^{4}H^{4}), and into carbonic oxide (CO).

While these gases are rising in hot condition, the air comes in contact with them and effects their combustion.  The dark portion, c, of the flame is where the carbon and gases have not a sufficiency of air for their thorough combustion; but gradually they become mixed with air, although not then sufficient for complete combustion.  The hydrogen is first oxidized or burnt, and then the carbon is attacked by the air, although particles of carbon are separated, and it is these, in a state of intense ignition, which produce the illumination.  By bringing any oxidizable substance into this portion of the flame, it oxidizes very quickly in consequence of the high temperature and the free access of air.  For that reason this part of the flame is termed the oxidizing flame, while the illuminating portion, by its tendency to abstract oxygen for the purpose of complete combustion, easily reduces oxidated substances brought into it, and it is, therefore, called the flame of reduction.  In the oxidizing flame, on the contrary, all the carbon which exists in the interior of the flame is oxidized into carbonic acid (CO^{2}) and carbonic oxide (CO), while the blue color of the cone of the flame is caused by the complete combustion of the carbonic oxide.  These two portions of the flame—­the oxidizing and the reducing—­are the principal agents of blowpipe analysis.

If we introduce a fine current of air into a flame, we notice the following:  The air strikes first the dark nucleus, and forcing the gases beyond it, mixes with them, by which oxygen is mingled freely with them.  This effects the complete combustion of the gases at a certain distance from the point of the blowpipe.  At this place the flame has the highest temperature, forming there the point of a blue cone.  The illuminated or reducing portion of the flame is enveloped outside and inside by a very hot flame, whereby its own temperature is so much increased that in this reduction-flame many substances will undergo fusion which would prove perfectly refractory in a common flame.  The exterior scarcely visible part loses its form, is diminished, and pressed more to a point, by which its heating power is greatly increased.