George Ernst Stahl
During the eighteenth century, the scientific theories that existed had no scientific basis. The scientists at the time, such as Aristotle and Stahl, had put forward theories based upon their own experiments. However they had never considered measuring the mass of the reactants or that of the products therefore their conclusions were based purely on qualitative observations and not quantitative ones.
In fact, Lavoisier's teacher, Rouelle, firmly believed that there were only four elements. This was a popular belief and the four elements were called 'The Four Elements of Aristotle'. These were as follows: Air, Earth, Fire and Phlogiston.
The latter was Stahl's theory. It was widely accepted and used as an explanation for the effects of burning a particular material or substance. According to Stahl's Theory, all metals, fire and coal were rich in Phlogiston. This invisible and inflammable 'element' provided an explanation for the fact that a candle extinguished when placed under a glass jar. More importantly, this theory explained why a metal could be transformed into a metal oxide when heated.
The chemical reaction which transformed a metal into the metal oxide was called 'Calcination'. By heating a metal, the metal lost Phlogiston and therefore transformed into metal oxide often in the form of a white powder.
METAL ----------------> METAL OXIDE
LOSS OF PHLOGISTON
Once oxidised, the metal had lost its lustre as well as its ductility. Thus the loss of Phlogiston caused the properties of the metals to change.
The opposite of Calcination was called 'Reduction'. This chemical reaction required coal: the coal restored the Phlogiston lost during Calcination and therefore transformed the metal oxide back into a metal.
METAL OXIDE ------------------------> METAL
GAIN OF PHLOGISTON
At first, this logical and plausible concept appeared to explain the effects of these chemical reactions on metals. However, shortly after Stahl's discovery, a scientist repeated Stahl's experiments and noticed a strange phenomena: The mass of the metal oxide was greater than that of the metal. This appeared to be impossible as the loss of Phlogiston during Calcination would suggest a decrease in mass.
Furthermore, other scientists had discovered that mercury oxide could be transformed into mercury without the use of coal ie. without a source of Phlogiston.
Stahl recognised the fact that there was a discrepancy between the experimental results and his theory, but he didn't attempt to explain this anomaly. However,
Louis Morveau, a chemist, suggested that Phlogiston had a negative mass and this concept was accepted by many scientists.
Despite popular belief in this new theory, Lavoisier was not convinced that Phlogiston existed as the Phlogiston theory appeared to contradict all the laws of conservation. Therefore Lavoisier decided to undertake his own experiments hoping to resolve the Phlogiston problem.