Carolyn McGregor, age 13, of Carson City, Nev., for her question:

WHO DISCOVERED OXYGEN?

Oxygen is a life supporting gas and a chemical element. It makes up about a fifth of the volume of air and is one of the most plentiful chemical elements on the earth. Oxygen was discovered by two chemists working independently: Carl Scheele of Sweden and Joseph Priestley of England.

Scheele's laboratory notes showed that he prepared oxygen between 1770 and 1773 by heating various compounds, including mercuric oxide. But Scheele's experiments were not published until 1775. He described how he prepared oxygen in 1774 by heating mercuric oxide.

Scheele called oxygen "fire air." Priestley called it "dephlogisticated air."

In 1777, the French chemist Antoine Lavoisier named this gas "oxygen." The word means "acid producer." Lavoisier and others had found that oxygen is a part of several acids.