Carl (or Karl) Wilhelm Scheele
Carl (or Karl) Wilhelm Scheele, (December 9, 1742 - May 21, 1786) a Swedish chemist, born in Stralsund, Pomerania, Germany, was the discoverer of many chemical substances, most notably discovering oxygen before Joseph Priestley. 

Scheele worked as a pharmacist in Stockholm, from 1770 to 1775 in Uppsala, and later in Köping.  His studies led him to the discovery of oxygen and nitrogen in 1772-1773, which he published in his only book, Chemische Abhandlung von der Luft und dem Feuer (Chemical Treatise on Air and Fire) in 1777, losing some fame to Joseph Priestley, who independently discovered oxygen in 1774.

Scheele also discovered other chemical elements such as barium (1774), chlorine (1774), manganese (1774), molybdenum (1778), and tungsten (1781), as well as several chemical compounds, including citric acid, glycerol, hydrogen cyanide (also known as prussic acid), hydrogen fluoride, and hydrogen sulphide. In addition, he discovered a process similar to pasteurization. 

Like many other chemists of his time, Scheele often worked under difficult and even dangerous conditions, which might explain his early death.

Scheele interpreted his experiments in the light of this phlogiston theory. He thought that hydrogen might be phlogiston, or perhaps a compound of phlogiston with some unknown substance, for it was clear that many metals could be used in the reaction 

Calx of metal + Hydrogen --> Metal + Heat. 

For example, when Scheele burned a hydrogen flame in a glass globe of air standing over water, he observed that the water rises. He reasoned, correctly, that this meant that combustion must use up the fire air (oxygen). His further interpretation of this experiment is an interesting piece of erroneous easoning. Since no product of this reaction was observed other than heat  water did not condense since hot water was used in his water bath), the  eaction taking place would have to be fire air + X --> heat. It is therefore reasonable to attempt to reverse the reaction, to decompose heat and produce fire air. To do this, one must heat a substance having a greater attraction for phlogiston than does fire air. Such a substance is nitric acid, because it readily attacks metals taking out their phlogiston: 

(Calx of M + X) + nitric acid --> Calx of M + (nitric acid + X) 

where (Calx of M + X) is equivalent to the metal M itself and (nitric acid + X) is taken as the red fumes of NO2

Then, since nitric acid boils away on heating, one must fix it by combination with potash: 

H+NO3-1 + KOH --> KNO3 + H2

and then distill it with oil of vitriol (H2SO4) in a retort: 

4 KNO3  +  H2SO4 ----> 4 NO2  +  O2  +  2 K2SO4  +  H2

If one then absorbs the NO2 gas in limewater, while the K2SO4 remains behind in the flask, an empty bladder can be filled with the fire air gas. This erroneous interpretation of a correctly planned experiment is worthy of some
thought on the part of the reader. 

Other methods of preparing oxygen were also used by Scheele, who interpreted the results of heating the red oxide of mercury according to the phlogiston theory using (fire air + X) as equivalent to heat and (calx of mercury + X) as equivalent to mercury metal: 

Calx of mercury + (X + fire air) <--> fire air + (calx of mercury + X) 

The doctrine of phlogiston did not fade until its replacement by the oxygen chemistry approach of Lavoisier and his colleagues around 1800.