Because some of the carbon escapes in a state of incandesence, and as soon as it reaches the air it combines with oxygen, and so forms carbonic acid gas.
If any dark body, such as the blade of a knife, be held between the eye and the flame of the candle, so as to shut off the light of the more luminous part, the pale fringe around the flame will be found distinctly perceptible. Incandesence means heated to whiteness.
"How oft is the candle of the wicked put out? and how oft cometh their destruction upon them?"—Job xxi.
250. Why does the flame terminate in a point?
Because cold air rushes towards the flame in every direction, and is carried upward. At the point where the flame terminates the cold currents have so reduced the temperature that combustion can no longer be sustained.
251. Why, if you hold anything immediately over the flame, will the flame lengthen?
Because, by preventing the rapid escape of the heated air, you maintain a temperature which increases the combustion at the point of the flame.