Combustion, Air, and Phlogiston
Georg Ernest Stahl
     We now know that the air we breathe is a mixture of gases, primarily oxygen and nitrogen. In the eighteenth century, however, the discovery that air was such a mixture, and the characterization of its components, was modern chemical research. Van Helmont's understanding that there were different gases with different chemical properties led to attempts to separate gases from air and react gases with air. These studies are the basis of our modern understanding of the nature of the atmosphere. 

The erroneous doctrine of phlogiston, introduced by Georg Ernest Stahl, enabled chemists to explain metal reduction and oxidation by the same mechanism.


     Georg Ernest Stahl

Although Scheele and Priestley did not use a symbol for phlogiston, it is convenient for us to write chemical reactions using symbols. If phlogiston is symbolized by X, the oxidation or rusting of iron as understood by Priestley would be written 

Iron --> Calx of iron + X. 

The reaction of carbon is similar, Carbon --> Calx of carbon + X. 

The reduction of the calx of tin can be, then, Calx of tin + X --> Tin, 

and the description of the actual smelting process of tin or iron is: 

Calx of tin + Carbon --> Tin + Calx of carbon 

Calx of iron + Carbon --> Iron + Calx of carbon