What is the name of the non-existent chemical once believed to be emitted when objects burned?

The non existent chemical once believed to be emitted when objects burned was called phlogiston. It was a substance supposed by 18th-century chemists to exist in all combustible bodies, and it was said to have been released in all acts of combustion.

Phlogiston was considered a fire-like element; it was said to be contained within things that were able to burn. It was held that it was actually released during the burning process. This was compared to the act of a person breathing. In fact, phlogiston means “burning up” in Greek.

The first person to mention it publically was German alchemist J. J. Becher. He wrote about it in his book Physica Subterranea; he called it “terra pinguis”. It was another German scientist George Ernst Stahl, who renamed the term “terra pinguis” to “phlogiston”. According to the theory of phlogiston, substances that could be burned were rich in phlogiston or were said to be “phlogisticated”. It was air that had the ability to absorb only a limited amount of phlogiston. When the air became phlogisticated, it could support neither burning, nor breathing.

The phlogiston theory became an outdated scientific theory by the end of the 18th century. It was found that the burning process required oxygen, a gas having a definite mass. This helped to explain why some metals, such as magnesium, gained mass instead of losing it while burning. Thus, the basis for a new scientific theory (the caloric theory) was formed.