The Phlogiston Theory

The following is mainly taken from wikipedia:

In 1667, Johann Becher published Physical Education. In it is found the first suggestion of what would later become the phlogiston theory. At Becher's time, alchemists believed that there were four classical elements: fire, water, air, and earth. In his book, Becher disputed this by eliminating fire and air and replacing them with three forms of earth. The third, terra pinguis, represented combustible properties. Becher believed that terra pinguis was a main feature of combustion which was released when substances capable of combustion were burned. Becher's theory was expanded in 1703 by a professor of medicine and chemestry called Georg Stahl who renamed terra pinguis phlogiston. The phlogistion theory as it is known today is mainly influenced by Stahl's representation.

The theory was an attempt of early scientists to explain the processes of burning, such as combustion. It states that phlogiston is a substance without color, smell, taste, or mass which is freed during the process of burning. A burned substance, now free of phlogiston, or "dephlogisticated", was considered to be in its true form. It was believed that oxygen was capable of absorbing only a certain amount of phlogiston, and that once it became fully phlogisticated it would no longer support the process of combustion. The fact that combustion stopped in an enclosed space supported this line of thought. Another relationship between oxygen and phlogiston that was believed was that phlogisticated air could not support life, since the job of air in the process of respiration was to remove phlogiston from the body.

Ironically, this early description was essentially opposite of oxygen's actual role in combustion.

Eventually, the phlogiston theory began to loose ground. One of the biggest observations that began this process was that when certain metals, such as magnesium, were burned, they actually gained weight. This was against phlogiston theory since burning was supposed to release phlogiston and make a substance lighter. Some tried to hold on to the theory anyway by suggesting that phlogistion actually had negative weight, others proposed that it was lighter than air, but these conjectures were proved false. During the eighteenth century, phlogiston began to be seen as a principle rather than an actual substance; when it was referred to at all, it was usually linked with hydrogen. Some scientists, most notably Joseph Priestely, held onto the concept of phlogiston theory throughout his career.