6.2.2 Fixed air
It was well known that ‘air’ was given off by magnesia (or limestone) when treated with acids. Black sought to show that this ‘air’, which he called ‘fixed air’ (carbon dioxide), is also lost when magnesia is heated. Hampered by practical difficulties in his efforts to collect the fixed air liberated during the heating of magnesia, Black used a series of chemical reactions to prove his argument. He dissolved the magnesia usta in sulphuric acid to produce a solution of Epsom salt. This solution was treated with fixed alkali (potassium carbonate), which precipitated magnesia. This regenerated magnesia, after being washed and dried, had the weight and the properties of the original compound.
As very little ‘air’ was given off during this sequence, the fixed air in the fixed alkali must have ended up in the magnesia. Black confirmed this by treating magnesia with sulphuric acid and then measuring the weight lost during this reaction, which was equal to the weight loss during calcination.
Black also noted that quicklime does not absorb ordinary air, but only the small quantity of fixed air contained in it. This implied that there were at least two chemically distinct ‘airs’, and Black knew that fixed air extinguished a candle. However, he was not interested in the chemical behaviour of gases, and although he carried out experiments which revealed that birds were unable to breathe in fixed air, he did not make any further contributions to the pneumatic chemistry he had so ably helped to found.)