During the seventeenth century, chemists needed a broad conceptual system to explain what caused the chemical reactions they observed. The phlogiston theory of combustion was the first comprehensive theory of chemistry and it served to satisfy the need for a broad system of explanation. This theory was based on phlogiston, a substance that chemists thought was contained in all combustible substances. The phlogiston theory was the first theory of chemistry that was able to explain a wide range of phenomena.
An example of a reaction that the phlogiston theory could explain is the preparation of metals from their ores by heating with charcoal (smelting). The transformation of an earthy substance, the ore, into a metal by this process appeared to be much the same whether the metal involved was iron, tin, or copper. The fact that metals became ores when heated and could be changed back into metals in the presence of charcoal was hard to reconcile without imagining the addition or subtraction of some substance. Therefore, it seemed plausible to assume that in each instance the ore, when heated with charcoal, took up a metallizing principle that conferred upon the earth the properties of a metal. This metallizing principle was first named terra pinguis (combustible earth) by Johann Becher in 1669. When metals were calcined, the terra pinguis escaped, leaving behind a metallic calx (what we today call an oxide). Ernst Stahl (1660-1734), in 1737, renamed Becher's terra pinguis phlogiston (from the Greek word for burning). It was almost exactly similar to terra pinguis, in that when a metal burned, it released phlogiston leaving a calx behind.
The phlogiston theory of combustion had strong explanatory power. During the smelting process, burning charcoal with the ore resulted in the metal. Since charcoal burns in air leaving only a small quantity of ash, it was clear that charcoal was rich in phlogiston. The reason a metal formed when its calx was heated with charcoal was therefore because the phlogiston left the charcoal and united with the calx.
It was easily observable that a burning substance would cease burning if the supply of air was cut off.Robert Boyle had already demonstrated in the mid-seventeenth century that combustion would not occur in a vacuum. The phlogiston theory of combustion was able to explain this by assigning to air the ability to absorb phlogiston. When the air was fully saturated with phlogiston, combustion ceased. Combustion, therefore, according to the phlogsiton theory, was completely impossible in a vacuum because there was no air to absorb the phlogiston.
Phlogiston was accepted by the 1770s despite some of its anomalous properties. Chemists could not satisfactorily explain why metals gained weight during combustion. If metals gave off their phlogiston during their combustion, should they not lose weight? Most chemists were not concerned with this detail because during the first half of the eighteenth century chemistry was not concerned with quantitative analysis. It was enough for most chemists that the phlogiston theory gave a good qualitative explanation of combustion. But towards the end of the eighteenth century, some chemists reconciled themselves to this gain in weight upon combustion by supposing that phlogiston had a negative weight, and therefore upon its expulsion from a metal on combustion would result in the metal getting heavier.
In 1772, Antoine-Laurent Lavoisier began working on the problem of combustion. He was not satisfied with the explanation of the weight gain of metals on combustion. Guyton de Morveau (1737-1816), a colleague of Lavoisier, had performed experiments showing that metals did without question increase in weight on combustion. Lavoisier used these results to propose his first theory of combustion--in all cases of combustion where an increase in weight is observed, air is absorbed, and that when a calx is burned with charcoal, air is liberated. This air he later called oxygen. This oxygen was what combined with metals during combustion and was what caused of the increase in a metal's weight on combustion.
Lavoisier's oxygen theory of combustion was almost the exact opposite of the phlogiston theory of combustion. In the phlogiston theory, phlogiston is released during combustion, and in the oxygen theory, oxygen is absorbed during combustion.
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