Polish alchemist, philosopher and physician Michael Sendivogius wrote about something in air that he called the "food of life", and this meant what we now call oxygen. Sendivogius found, between 1598 and 1604, that the substance in air is the same as he got by heating potassium nitrate. Some people believe this was the discovery of oxygen while others disagree. Some say that oxygen was discovered by Swedish pharmacist Carl Wilhelm Scheele. He got oxygen in 1771 by heating mercuric oxide and some nitrates. Scheele called the gas "fire air", because it was the only gas known to allow combustion (gases were called "airs" at this time). He published his discovery in 1777.