One important observation that supporters of th phlogiston theory had trouble explaining was that the calx of a metal weighs more than the metal itself .
The phlogiston theory explains about the combustible materials which releases undetectable substance (phlogiston) when they burn. Slightly combustible materials like metals release very little phlogiston and forms metal calx, which weighs more than the metal when they burn. When a metal burns it converts into metal oxide, which weighs more than the metal. The supporters of the phlogiston theory (phlogistonists) explains that phlogiston had negative mass, hence metal releases very little phlogiston. But, this was denied by the critics.
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