Crucial to disprove the phlogiston hypothesis was the observation that magnesium gains mass when burns.

But how was this observation made in the 18th century? Is there any way this property of magnesium would be obvious to a layman today?

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    $\begingroup$ Presumably by weighing magnesium before and after it was burnt? It's not that difficult to do. $\endgroup$
    – bon
    Mar 9 '16 at 14:17
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    $\begingroup$ Per Wikipedia "The metal itself was first produced by Sir Humphry Davy in England in 1808." So burning the metal would have taken place in the early 19th century. en.wikipedia.org/wiki/Magnesium#History "Phlogiston remained the dominant theory until the 1780s when Antoine-Laurent Lavoisier showed that combustion requires a gas that has mass (oxygen) and could be measured by means of weighing closed vessels." en.wikipedia.org/wiki/Phlogiston_theory#Challenge_and_demise So by the time metallic magnesium was isolated the phlogiston hypothesis was dead. $\endgroup$
    – MaxW
    Mar 10 '16 at 4:28

The phlogiston hypothesis is about a substance, phlogiston, which was supposed to exist within matter that be consumed or released when it is combusted. If this were true, then when magnesium is combusted the mass would decrease.

To someone who believed this to be true, it would come as a great surprise to find that if they combusted 100g of magnesium the mass would increase by about 65%. This would be easy to measure with a set of balances, even with much smaller masses of a few grams.

A layman might not come across many day-to-day examples of a combustion reaction with a mass increase, since most things that burn release a lot of mass as gases, and that effect is what phlogiston was intended to explain. One possibility, aside from burning metal, is the oxidation of metal in air. Someone could weigh an iron nail or other object, allow it to rust, and weigh again to show it has gained mass. This could easily be demonstrated by any merchant with a set of scales, but it's not obvious the rust is an oxide related to the combustion products.

  • $\begingroup$ Unless of course phlogiston had negative mass, which was an initial hypothesis to explain these observations. $\endgroup$
    – long
    Mar 10 '16 at 2:32

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