"Black discovered the difference between weak and strong s, i.e. between the alkali s and s. The work... is essentially quantitative, and... leads into... gas analysis, which was to play a very important role in providing a new chemical theory. ...Black ...first showed that and were two different substances. Although both effervesce when treated with s... [m]agnesium carbonate does not form common lime when heated strongly, and on cooling, the residue is insoluble in water. This product of ignition (oxide) however forms the same salts with acids as does the original salt (carbonate) with the difference that no effervescence occurs. Black also observed that during ignition "air" () is lost, and supposed that to be responsible for the loss of weight as well as for the effervescence... [H]e dissolved the magnesium oxide in sulphuric acid and then precipitated the magnesium with . ...[T]he composition of the precipitate was identical with that... before the ignition... He... concluded that alkali carbonates were not elemental substances, as had been originally thought, because they give "air" to the... oxide... [the] same "air"... responsible for the effervescence... Black then [examined] lime and limestone and applied similar experiments. He established that the air was not identical with atmospheric air... only a component... called by Black "fixed air"... that part... absorbed by lime and the alkali hyroxides. ...[T]he relationship... is similar to that between alkalis and acids... alkalis are "in some measure neutralized" by the fixed air. However, the relation between acids and alkalis is stronger as the acid drives out the fixed air."