How did Joseph Black show that fixed air differed from ordinary atmospheric air?

Joseph Black demonstrated that "fixed air" (now known as carbon dioxide) was a distinct chemical substance rather than merely atmospheric air in a different state of purity through a series of rigorous, quantitative experiments.1

By using a balance to measure weight changes, Black observed that when magnesia alba or chalk (calcium carbonate) were heated, they lost weight as they released a gas, transforming into "usta" forms (such as quicklime) that would no longer effervesce with acids. He proved this gas originated from the compounds themselves—not from the furnace fire—by showing that the original substance could be reconstituted by reintroducing the gas via a potash solution.1

This work established that gases could be unique chemical substances, fundamentally shifting the scientific understanding of the era, which had previously viewed all gases as variations of atmospheric air.1