What did Joseph Black discover about slaked quicklime exposed to the atmosphere?

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Joseph Black Joseph Black, detail of an engraving by J. Rogers after a portrait by Sir Henry Raeburn

Joseph Black discovered that when slaked quicklime (hydrated calcium oxide) is exposed to the atmosphere, it becomes neutralized because it absorbs a specific component of the air, which he identified as carbon dioxide. Black famously termed this gas "fixed air."1

This finding was significant because it demonstrated that the gas absorbed from the atmosphere was chemically distinct from common air, rather than being atmospheric air in a different state of purity. This work helped establish the field of pneumatic chemistry and proved that gases could be treated as individual chemical substances.21