In his M.D. thesis, Black explores reactions of magnesium salts by carefully weighing his samples before and after reactions. He heated magnesia alba (hydrated MgCO3) strongly and found it lost about half its mass. The gas lost he called “fixed air” – now known as carbon dioxide (CO2). On reflecting that his thesis did not contain enough medicine, he included a section on stomach acid and the use of magnesia as an antacid.