83
Priestley’s apparatus for his gas
experiments appear in his book about
his discoveries. At the front, a mouse is
kept in oxygen under a jar; on the right,
a plant releases oxygen in a tube.EXPANDING HORIZONS
another, he found that it made a
refreshing sparkling drink, which
later led to the craze for soda water.
Releasing oxygen
On August 1, 1774, Priestley first
isolated his new gas—which we
now know as oxygen (O 2 )—from
mercuric oxide in a sealed glass
flask by heating it with sunlight
and a magnifying glass. He later
discovered that this new gas kept
mice alive much longer than
ordinary air, was pleasant to
breathe and more energizing than
ordinary air, and supported the
combustion of various substances
he burned as fuel. He also showed
that plants produce the gas in
sunlight—a first hint of the process
we call photosynthesis. At the time,
however, combustion was thought
to involve the release from a fuel
of a mysterious material called
phlogiston. Because this new gas
did not burn, and therefore must
contain no phlogiston, he called it
“dephlogisticated air.”
Priestley isolated several other
gases at about this time, but then
went on a European tour, and did
not publish his results until late the
following year. Swedish chemist
Carl Scheele had prepared oxygen
two years before Priestley, but did
not publish his results until 1777.
Meanwhile in Paris, Antoine
Lavoisier heard of Scheele’s work,
was given a demonstration by
Priestley, and promptly made his
own oxygen. His experiments on
combustion and respiration proved
that combustion is a process of
combining with oxygen, not
liberating phlogiston. In respiration,
oxygen absorbed from the air
reacts with glucose and releases
carbon dioxide, water, and energy.
He named the new gas oxygène, or
“acid-maker,” when he discovered
that it reacts with some materials—
such as sulfur, phosphorus, and
nitrogen—to make acids.
This led many scientists to
abandon phlogiston, but Priestley,
though a great experimenter, clung
to the old theory to explain his
discoveries and made little further
contribution to chemistry. ■The most remarkable of
all the kinds of air I have
produced...is, one that is
five or six times better than
common air, for the purpose
of respiration.
Joseph PriestleySee also: Joseph Black 76–77 ■ Henry Cavendish 78–79 ■
Antoine Lavoisier 84 ■ John Dalton 112–13 ■ Humphry Davy 114
Joseph Priestley
Born on a farm in Yorkshire,
Joseph Priestley was brought
up as a dissenting Christian,
and was intensely religious
and political all his life.
Priestley became
interested in gases while
living in Leeds in the early
1770s, but his best work
was done after he moved to
Wiltshire as librarian to the
Earl of Shelburne. His duties
were light and left him time
to conduct research. He later
fell out with the earl—his
political views may have been
too radical—and in 1780, he
moved to Birmingham. Here
he joined the Lunar Society,
an informal but influential
group of freethinkers,
engineers, and industrialists.
Priestley’s support for the
French Revolution made him
unpopular. In 1791, his house
and laboratory were burned
down, forcing him to move to
London and then to America.
He settled in Pennsylvania,
and died there in 1804.Key works1767 The History and Present
State of Electricity
1774 –77 Experiments and
Observations on Different
Kinds of Air