A Milestone in Science History: Lavoisier's Experiment

Before the mid-18th century, it was widely believed that air was a single "element." The great chemistLavoisier (A.-L. Lavoisier) used precise balance measurements and ingenious logic to completely overturn this notion.

Lavoisier continuously heated mercury in a sealed retort for 12 days. He observed a red powder (mercuric oxide) forming on the silvery mercury's surface, and the liquid level in the bell jar rose by about 1/5. This quantitative approach — "replacing the eye with the scale" — not only revealed air as a mixture but also gave birth to modern chemistry. The core reaction equation is:

$$\text{Mercuric Oxide} \xrightarrow{\text{heat}} \text{Mercury} + \text{Oxygen}$$

The "Duet" of Nitrogen and Oxygen

Lavoisier ultimately confirmed that air is composed of about 1/5oxygen(which supports respiration and combustion) and about 4/5nitrogen(which does not support combustion). This discovery marked chemistry's transition from alchemical conjecture to a rigorous, quantitative science.