Joseph Priestley and Antoine Lavoisier played a leading role in the emergence of modern
chemistry. In a series of experiments on chemical reactions and combustion, Lavoisier was
able to measure properties of things invisible to him (gases), discovering the law of
conservation of mass. This law is one of the most important achievements of science and is
the basis to understand chemical reactions. Lavoisier systematically conducted Philo’s
candle experiment (Lavoisier 1777, p. 195), concluding that any change in volume is well
below 1%. Lavoisier concluded correctly that the change of volume in the typical candle
experiment was due to the thermal expansion of air. In a simple experiment, he placed an
inclined inverted glass bell partially submerged in mercury, ignited the candle and quickly
moved the candle inside the bell. The bell was quickly submerged and the column of
mercury prevented air to bubble out, but strictly speaking this is not a closed system
because initially air can enter or leave the bell. In contrast, his contemporary and scientific
adversary Joseph Priestley in his experiments on combustion used sunlight and a lens to
ignite different substances inside a closed volume. Lavoisier also designed a closed volume
variation of the candle experiment: he put phosphorous near the candle wick, and closed
the volume with a mercury bath, extracting some air to get a column of mercury that sealed
the volume and allowed the gas to expand without bubbling out of the vessel. He then
introduced a heated piece of iron through the column of mercury to ignite the phosphorous
and light the candle.