The discovery of oxygen was given meaning by the clear revolutionary mind of Lavoisier, who perished in the French Revolution.
Lavoisier repeated an experiment of Priestley's, which is almost a caricature of one of the classical experiments of alchemy.
Both men heated the red oxide of mercury using a burning glass - the burning glass was fashionable just then - in a vessel in which they could see gas being produced and could collect it.
The gas was oxygen.
That was the qualitative experiment, but to Lavoisier it was the instant clue that chemical decomposition could be quantified.
The idea was simple and radical:
Run the experiment in both directions and measure the quantities exactly.
Burn mercury so that it absorbs oxygen, and measure the exact quantity of oxygen that is taken up between the beginning of the burning and the end.
Now turn the process into reverse.
Take the mercuric oxide that has been made, heat it vigorously and expel the oxygen from it again.
Mercury is left behind, oxygen flows into the vessel, and the crucial question is:
How much?
Exactly the amount that was taken up before.
Suddenly the process is a material one.
Essences, principles, phlogiston have disappeared, two concrete elements have really been put together and taken apart.
After the fire, the sulphur, the burning mercury, it was inevitable that the climax of the story should take place in the chill damp of Manchester.
Here, between 1803 and 1808, a Quaker schoolmaster called John Dalton turned the vague knowledge of chemical combination, brilliantly illuminated as it had been by Lavoisier, suddenly into the precise modern conception of atomic theory.
It was a time of marvellous discovery in chemistry.
In those five years, ten new elements were found.
And yet, Dalton was not interested in any of that.
He was, to tell the truth, a somewhat colourless man.
He was certainly colour-blind.
He was a man of regular habits, who walked out every Thursday afternoon to play bowls in the countryside.
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