Lavoisier while performing experiments with phosphorus and sulfur observed that when these substances were ignited, they absorbed large quantities of air. He performed these experiments under a bell jar and noted that the total weight of the jar and sulfur was the same before and after the experiment. But the weight of the sulfur increased. This increase must be at the expense of air contained under the bell jar.

Lavoisier reported "I discovered that sulfur in burning, far from losing weight, on the contrary gains it... from a pound of sulfur one obtains much more than a pound of vitriolic acid...it is the same with phosphorus."

Through these experiments he showed that combustion did not result from a release of phlogiston but from absorption of air and weight. Thus he proved that the Phlogiston Theory false.

Lavoisier did repeated experiments with mercury, tin, and lead calx. He heated these metals in a sealed airtight vessel. He noticed that total weight of this vessel remained the same before and after burning. When the seal of the vessel was removed, the vessels filled with more air and an increase in weight was found. Lavoisier was sure that the increase in weight was a result of air because the increase in weight of the metal after burning was almost the same as the increase in weight of the vessel after the seal was removed.

Based on this observations, Lavoisier came to the conclusion: The air of the atmosphere was not a simple substance, but a mixture of two different substances. One of these was respirable, supported combustion, and made possible rusting; the other was not respirable, did not rust metals, and did not support combustion.

In 1778 Lavoisier reported his findings on air. He reported that the gain in weight of metals was due to "nothing other than the healthiest and purest part of the air." The air released by heating mercury calx was "purer than the very air in which we live." When mercury calx was reduced with charcoal, "fixed air" (what we call carbon dioxide today) was given off. This was due to the combination of charcoal with "pure part of the air."

Lavoisier also explained some of his earlier experimental findings. When phosphorus was burned it combined with "pure air" to form "acid of phosphorus," and sulfur formed "vitriolic" acid. He named this air "oxygen" from Greek words meaning "to give birth to acid". This is how oxygen got its name.