How did Joseph Black show that magnesia and lime were different substances?

In 1755, the Scottish chemist Joseph Black demonstrated that magnesia (magnesium oxide) and lime (calcium oxide) were distinct substances through a series of quantitative experiments.12

Black observed that while both substances behaved similarly when reacting with acids, they differed significantly in their physical properties. Specifically, he noted that magnesia produced a soluble sulfate when treated with acid, whereas the sulfate derived from lime was insoluble. Furthermore, Black found that unlike quicklime, magnesia was neither caustic nor soluble in water.21

His research also involved heating these compounds and measuring weight changes. He discovered that heating magnesia alba (magnesium carbonate) released a gas he termed "fixed air," which could be reintroduced to the remaining magnesia usta (magnesium oxide) using potash to recreate the original substance. These experiments were foundational in establishing that gases could be distinct chemical substances rather than merely atmospheric air in different states of purity.1