I have looked this theory up on google and found that the theory is that when metal is burnt calx (metal oxide) and phlogiston is created (metal --> calx + phlogiston). Then I found out that phlogiston is a substance supposed by 18th century scientists to exist in all combustible bodies and to be released during combustion. But it later got disproved by Antoine Lavoisier who proved that metal and oxygen when burnt produced calx.

So my questions are:

  1. What exactly is the phlogiston theory and if my understanding of it correct?
  2. What exactly did Georg Ernst Stahl (discovered the phlogiston theory) think phlogiston was?
  3. Why did the 18th century scientists believe that phlogiston existed in all combustible bodies and why not just a product created during the combustion reaction?
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    $\begingroup$ 1.)"Phlogiston" is, as far as I know, not a scientific theory, because it does not allow (or even attempt) to make any quantitative predictions. A philosophical setup much like Demokrit's atoms chemistry.stackexchange.com/questions/74227/… , only it's actually wrong. $\endgroup$
    – Karl
    Aug 1, 2019 at 19:58
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    $\begingroup$ Phlogiston has a negative mass! Metallurgists of the 18th century certainly knew that the calx of a metal weighed more than the base metal. Ironically, Priestley showed that a sealed vessel containing HgO formed Hg but did not change its weight but was, nonetheless, a proponent of the phlogiston theory. $\endgroup$
    – user55119
    Aug 1, 2019 at 20:07
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    $\begingroup$ The phlogiston theory and some of its history have been discussed over at the History of Science and Mathematics stack exchange community. $\endgroup$
    – Ed V
    Aug 1, 2019 at 20:10
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    $\begingroup$ @JonCuster Well, there is a history-of-chemistry tag! I see nothing wrong with this question. $\endgroup$
    – Buck Thorn
    Aug 1, 2019 at 21:42
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    $\begingroup$ @BuckThorn - perhaps more for meta, but at some level I think there is a limit to what on-topic chemical history consists of. History of how currently accepted chemistry came to be, yes. Why any of a myriad of discarded ideas arose or were discarded, not so much. While interesting, the second are more for History of Science and Mathematics to my taste. $\endgroup$
    – Jon Custer
    Aug 1, 2019 at 23:56