Sentence:  The substance to which all common flames owe their brightness is finely-divided charcoal. ... Of this formation of charcoal the proof is obvious whenever a candle <WSD>flares</WSD> and smokes; for the unburnt charcoal soon collects in the upper part of the flame, and if not removed is apt to fall into the cup of the candle, where it forms a kind of second wick, rapidly melting away the tallow, and disfiguring the candle, ... 
Meaning of the word marked with <WSD> tags:  To shine out with a sudden and unsteady light; to emit a dazzling or painfully bright light.