Many alchemists, such as Cornelius Drebbel, the Dutch inventor who in 1608 had suggested production of oxygen by heating saltpeter, had noted the existence and properties of oxygen before Scheele, investigating the air in 1772, demonstrates that air is a mixture of two gases.

He calls one of these “fire air” because it supports combustion, and the other “foul air”, because it is left after the fire air has been used up.

Scheele obtains the “fire air” by heating potassium nitrate, mercury dioxide, and many other substances.