Stahl · Phlogiston theory
What escapes from something burning?
That time, that place
In the early eighteenth century, the great question chemistry had to answer was burning. Wood burns down to a little ash, far lighter than it began. Something appears to have left.
Why this question
Stahl gave that departing thing a name: phlogiston. What burns well is rich in it, and ash is the husk left when it has gone.
What was found
The theory explained a remarkable amount.
Burn a metal and it turns to a crumbly calx. Heat that calx hard with charcoal and metal returns — the charcoal's phlogiston has moved across. A candle sealed in a jar soon goes out: the trapped air can take up no more phlogiston.
For nearly a century it was the framework of chemistry.
One thing nagged. Burn a metal and it gets heavier. If something left, why the gain? Some proposed that phlogiston has negative weight.
Fire was one of the four elements
Recovering metal by heating its calx with charcoal
It collapsed once Lavoisier began weighing precisely before and after