How do we know carbon dioxide has carbon in it?

Experimental evidence to show that carbon is present in carbon dioxide. The carbon dioxide is generated using carbonate and acid. Next a strip of Mg is ignited and burns in the generated carbon dioxide. A 2nd strip of Mg is burned in air to contrast with the first.
The first product contains dark spots which are elemental carbon.
The second is just white ash which is magnesium oxide (and maybe some magnesium nitride).

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