By 1770, Antoine Lavoisier was carefully studying how combustion worked.
How did things burn and how did they rust? Stahl’s phlogiston theory was still accepted, but Lavoisier had a problem with it: the theory had never been proven with measurements.
Two hundred years before, Galileo had stressed the importance of measurement in astronomy.
Lavoisier wanted to apply that to chemistry. He started burning many different substances and weighing the results.
Wood turned to ash and was much lighter. Candles burned and left nothing behind. He even burned a diamond and watched it seemingly vanish. Where did the disappearing matter go?
Tin, sulfur, and phosphorus all seemed to gain weight when burned. Lavoisier then studied rust and found that metals were heavier when they rusted. Where did the extra matter come from?
Even the alchemists knew that burning substances created gases. Lavoisier devised an apparatus with sealed containers where he could capture gases released during combustion and measure their weight.
In this way, he could also measure the amount of gas absorbed as a result of rust.
He placed a piece of wood in the sealed container and weighed it. Then he burned the wood until it was ash and weighed the container again.
This time the weight was the same. He burned tin, sulfur, and phosphorus with no weight change. He allowed a piece of iron to rust in the container and weighed it. Again, the weight did not change.
Lavoisier had discovered chemistry’s first great law, the law of the conservation of matter. This law states that matter can neither be created nor destroyed; it can only change from one form to another.
For example, when wood is burned, the amount of solid weight that is lost when the wood turns to ash is exactly equal to the amount of gas weight added to the air.
The law is the basis of all present-day chemical formulas.