The Science Book

82

DEPHLOGISTICATED

AIR

JOSEPH PRIESTLEY (1733–1804)

F

ollowing Joseph Black’s

pioneering discovery of

“fixed air,” or carbon dioxide

(CO 2 ), an English clergyman named

Joseph Priestley became interested

in investigating various other “airs,”

or gases, and identified several

more—most notably oxygen.

While a minister in Leeds,

Priestley visited the brewery close

to his lodgings. The layer of air

above the brewing vat was already

known to be fixed air. He found that

when he lowered a candle over the

vat, the candle went out about 12 in

(30 cm) above the froth, where the

flame entered the layer of fixed air

floating there. The smoke drifted

across the top of the fixed air,

making it visible and revealing

the boundary between the two airs.

He also noticed that the fixed air

flowed over the side of the vat and

sank to the floor, because it was

denser than “ordinary” air. When

Priestley experimented with

dissolving fixed air in cold water,

sloshing it from one vessel to

IN CONTEXT

BRANCH

Chemistry

BEFORE

1754 Joseph Black isolates the

first gas, carbon dioxide.

1766 Henry Cavendish

prepares hydrogen.

1772 Carl Scheele isolates a

third gas, oxygen, two years

before Priestley, but does not

publish his findings until 1777.

AFTER

1774 In Paris, Priestley

demonstrates his method to

Antoine Lavoisier, who makes

the new gas and publishes his

results in May 1775.

1779 Lavoisier gives the gas

the name “oxygène.”

1783 Geneva’s Schweppes

Company starts making the

soda water Priestley invented.

1877 Swiss chemist Raoul

Pictet produces liquid oxygen,

which will be used in rocket

fuel, industry, and medicine.

As Priestley discovers,

oxygen is separate from

“fixed air” (carbon dioxide).

Oxygen does not burn,

so it cannot contain the fire

element phlogiston.

Oxygen is

dephlogisticated air.

So combustion is a process of

combining with oxygen.

Phlogiston

does not exist.

But Lavoisier shows that other

gases and materials burn

readily in oxygen.