The Phlogiston Theory

Burning has been important to people ever since early humans learnt how to control fire. But HOW do things burn? This was a question which it took centuries to answer - and until it could be answered, chemistry could never make much progress. By the 17th century scientists were beginning to realise that the burning of fuels, the reactions of metals in air and the breathing of animals all had something in common - they were all faster or slower versions of the same type of reaction.

Wood turns to ashes when it burns. Some metals become soft powders when they are heated - and can then be changed back to metals by heating them again with charcoal. To the 17th century scientists Johann Becher (1625 - 1682) and Georg Stahl (1660 - 1734) there was a clear explanation for observations like these. Burning could be explained by the removal of a substance - and that substance they called phlogiston. Stahl developed this idea, which became known as the Phlogiston Theory and which was recognised as the explanation for burning for around 100 years. There were lots of problems with the theory - but it was better than no theory at all! At this stage no-one was carrying out work in chemistry which involved accurate measurements, so phlogiston was the theory of the day.

What was the Phlogiston theory?
According to the phlogiston theory -

• metals and all other combustible substances contain a substance known as phlogiston which is released into the air on burning along with caloric (heat)
• air has a limited ability to absorb phlogiston
• phlogiston cannot be detected by the senses

In its day the phlogiston theory was used to explain lots of observations about combustion. These explanations sometimes sound a bit far-fetched to our modern ears. For example, here are some explanations using the phlogiston theory -

• substances lose mass when they burn because they lose phlogiston
• a flame goes out in a container because the air becomes saturated with phlogiston
• charcoal leaves hardly any ash when it burns because it is almost pure phlogiston
• a mouse dies in an airtight container, or in a container where a candle has been burnt until it goes out, because the air is saturated with phlogiston
• some metal ashes turn back to metals when heated with charcoal because the charcoal restores the levels of phlogiston in the ash

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Antoine Lavoisier - the first 'modern' chemist

Joseph Priestley - a lucky dabbler