The first step toward a theory of chemical reactions was taken by Georg Ernst Stahl in 1697 when he proposed the phlogiston theory, which was based on the following observations.
Metals have many properties in common.
These calxes are not as dense as the metals from which they are produced. Some of these calxes form metals when heated with charcoal. With only a few exceptions, the calx is found in nature, not the metal. These observations led Stahl to the following conclusions. Wood and charcoal are particularly rich in phlogiston because they leave very little ash when they burn.