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HISTORY OF SCIENCE AND TECHNOLOGY
Antoine Lavoisier and the demise of the phlogiston theory
It was Antoine Lavoisier, working in Paris, who used accurate measurements and deductive reasoning to raise serious doubts about the phlogiston theory. He completed the series of experiments shown below.
Experiment 1: Heating Mercury in atmospheric air
A diagram of the apparatus used by Lavoisier
Lavoisier used 4 ozs (113.4 grammes) of mercury. There were exactly 50 cubic inches (819.35 cm3) of air in the bell jar.
The furnace was ignited and the apparatus was heated. There was no change observed during the first 24h but after 48h Lavoisier observed red spots on the surface of the mercury.
After a few days of continual heating the surface of the mercury was completely covered in red.
A diagram of the apparatus at the end of the experiment.
Once the apparatus had cooled down, Lavoisier took the new measurements.
He discovered that 8 cubic inches of gas weighing exactly 3.5 grains (0.23 grammes) had "disappeared" from the bell jar.
The mercury + calx in the retort flask has gained exactly 3.5 grains.
When he tested the gas left in the bell jar he found that it had lost its "active" ingredient. It no longer supported combustion or respiration.
Lavoisier concluded that the Mercury had taken this "active gas" from the air.
Experiment 2: It was Joseph Priestley's observations on heating mercury calx that gave Lavoisier the information he needed to move on. According to Priestley
Lavoisier carefully removed all of the calx from the surface of the mercury from Experiment 1. He weighed the calx to make sure that he had the full 3.5 grains.
The mercury calx was placed in a retort flask over the furnace. The apparatus was assembled as in the first experiment but the liquid in the bell jar was brought up to a higher level.
During this experiment Lavoisier's observations were the same as those of Priestley. A grey mist appeared in the retort flask and drops of mercury could be seen running down on the inside.
When all of the red calx had disappeared he let the apparatus cool down before taking any measurements.
The volume of the gas in the bell jar had increased by 8 cubic inches and and the weight by 3.5 grains. Tests on the gas in the bell jar showed that it was "active" - it supported combustion and respiration.
Finally Lavoisier weighed the mercury in the retort flask. It had lost exactly 3.5 grains in weight.
Because the "active gas" produced an acidic gas after a candle had burnt in it (tested with litmus paper), Lavoisier named the "active gas" oxygen (from the Greek meaning acid producer).
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