1667

Phlogiston

Robert Boyle (1627–1691), Johann Joachim Becher (1635–1682)

Phlogiston does not exist. It was supposed to be the essence of fire—an obsolete concept that might seem strange to us now. Things that burned easily were believed to have lots of phlogiston in them, and what we call fire was just the phlogiston being released. This theory was first put forward by the German alchemist and adventurer Johann Joachim Becher in his book Physica subterranea (1667), and by the early 1700s it was widely believed.

By the 1600s, it was well known that after things burned, they had lost a good deal of weight, and Becher attributed this to the loss of their phlogiston. The more they burned, the more phlogiston they had to lose in the first place. Air could only absorb a certain amount of it because, if you tried to enclose a fire, it soon went out—apparently choked by excess phlogiston gathering around it and keeping the rest from exiting. This “phlogisticated” air was known not to support life, so it was also believed necessary for animals to exhale phlogiston, which was not possible in air that was already saturated with it.

Becher’s theory was an honest and reasonable attempt to explain the facts of combustion, which no one understood, and his description of phlogiston fit the facts as well as or better than any other theory. It also related combustion to respiration, which was quite correct in principle. There were only a few problems with the idea, but as the years went on, it became clear that they would not go away. One of the biggest difficulties was that some metals apparently gained weight when they burned—a fact much emphasized by chemist Robert Boyle. There was much arguing over this point, but over time the evidence became incontrovertible. Some advocates of Becher’s theory suggested that phlogiston had negative weight, but that idea created more problems than it solved. The final blow to phlogiston came around 1774 with the discovery of oxygen, which flipped over the understanding of fire completely: it wasn’t a loss of phlogiston but rather a gain of oxygen.

SEE ALSO Carbon Dioxide (1754), Oxygen (1774)

Phlogiston being released—if there were such a thing as phlogiston. The theory made more sense than it is given credit for, but it collapsed under the weight of too much contradictory evidence.