Happy Birthday, Joseph Priestly
Breathe deep! With each breath, your lungs “discover” oxygen!
Animals need oxygen to burn food and gain energy. It is plants who put the oxygen we need into the air; plants separate oxygen atoms from hydrogen atoms during photosynthesis (in other words, they split water into its component parts) and then release the oxygen as a waste gas.
This means that animals are dependent on plants for both food and oxygen. This is why plants must vastly outnumber animals on the planet.
But just because we need oxygen doesn't mean that we know much about it. Look around you—sniff the air—notice that oxygen is odorless, tasteless, colorless, and invisible—just like nitrogen and carbon dioxide and many other gases!
Scientists and naturalists “discovered” oxygen when they were able to separate it out from other odorless, invisible gases in the atmosphere. The guy that we usually give credit for this discovery, Joseph Priestly, was born on this day in 1733.
Priestly did a lot of experiments and tried to separate out different “airs.” In one of those experiments, he did manage to isolate oxygen—but he didn't know that he had done that per se. He thought he had removed from air an invisible fluid named phlogiston, which (he thought) caused heating and burning.
Priestly called his discovery "dephlogisticated air." He published his findings so that other scientists could copy his apparatus and experiments and compare their results.
That part—publishing his work in specific detail so that others could replicate it—is very good. It is what happens in science—and it is the reason that Priestly gets credit for discovering something he didn't know he'd discovered. (Two scientists, Antoine Lavoisier and Carl Wilhelm Scheele, also isolated oxygen but were either slower to do so or slower to publish—so they don't usually get the credit.)
What isn't so good for Priestly's reputation is that he held onto an incorrect theory even when evidence came to light that pointed in another direction. The whole phlogiston theory was wrong, but Priestly was determined to defend it and fought against the oxygen theory of combustion.
By the way, Priestly also invented soda water and wrote a history of electricity. You might think that, with all this scientific discovery and invention, he would be referred to as a scientist. But although he contributed to science, he is generally called a theologian (one who studies religion), a cleric (a person in a religious order), a “natural philosopher,” and a teacher.