In 1667 the German chemist Johann Joachim Becher (1635–1682) published a book called Physica Subterranea (Physics below ground), in which he revised the traditional view of the classical elements. Becher replaced the elements fire and earth with three alternative forms of earth to which he gave Latin names: terra lapidea, or “stony earth,” which was the quality allowing earth to fuse into a solid mass, terra fluida, or “flowing earth,” governing the ease with which earth will flow, and terra pinguis, or “fatty earth,” which is concerned with combustion. Becher argued that when any combustible substance is burned terra pinguis is released.

One of Becher's students was another German chemist Georg Ernst Stahl (1660–1734). Stahl expanded Becher's ideas and renamed terra pinguis, calling it phlogiston, from the Greek word phlogos meaning “flame.” Phlogiston was colorless, odorless, tasteless, and could not be sensed by touch, but it possessed mass. Every combustible substance contained it and released it when it was burned, together with caloric (heat). The residue, after burning, was called calx (plural calces). Calx was the true form of the substance.

Prior to burning, a substance was said to be “phlogisticated” and after burning the calx was “dephlogisticated.” Calx weighed less than the original phlogisticated substance because it had lost phlogiston. Different substances left behind different amounts of calx depending on the amount of phlogiston they contained. Charcoal and sulfur leave very little calx because they are almost pure phlogiston.

Burning released phlogiston into the air, and when substances were burned in an enclosed space the air became increasingly phlogisticated. A point could be reached where the phlogisticated air was incapable of supporting further combustion because it was saturated with phlogiston. What is more, animals could not survive in fully phlogisticated air, because respiration became impossible. Respiration, therefore, removed phlogiston from the body.

The phlogiston theory was highly successful because for more than a century it provided a plausible explanation for natural phenomena and experimental results. Chemists believed it, and it was not until late in the 18th century that it was finally disproved and quickly abandoned.