What extinguishes candles is the invisible gas that falls onto the flame. This gas displaces the oxygen that the candle needs to stay lit, because it is denser than the atmosphere and falls until it reaches the bottom of the glass. Indeed, the combustion of the candle is a chemical reaction. For this to occur, wax, oxygen and heat are needed. If one of these elements is missing, there is no combustion. Density has been important in experiments, such as when carbon dioxide sank to the bottom of the glass because it is denser than oxygen.