"We must trust to nothing but facts: these are presented to us by Nature, and cannot deceive," wrote Lavoisier.

When he meticulously weighed different materials after they were combusted, h e noticed a consistent theme: they gained mass. To Lavoisier this observation implied that combustion was a process of transformation: the merger of material with air to create a n ew material. "Nothing is lost, nothing is created, everything is transformed," claimed Lavoisier. 

A year later he published a memoir on the subject unequivocally demonstrating that the new gas--that he now called oxygen--was indeed the active component in the air that reacted during combustion. The publication, in a single sweep, spectacularly demolished phlogiston theory and sparked the "chemical revolution" that ushered in modern chemistry. 

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