The Science Book

79

The first hydrogen balloon, inspired

by Cavendish, was cheered by a huge

crowd of spectators. Since hydrogen is so

explosive, modern balloons use helium.

See also: Empedocles 21 ■ Robert Boyle 46–49 ■ Joseph Black 76–77 ■ Joseph Priestley 82–83 ■
Antoine Lavoisier 84 ■ Humphry Davy 114

EXPANDING HORIZONS

Explosive discoveries
Cavendish also mixed measured
samples of his gas with known
volumes of air in bottles, and
ignited the mixtures by taking
the tops off and applying lighted
pieces of paper. He found that with
nine parts of air to one of hydrogen
there was a slow, quiet flame; with
increasing proportions of hydrogen
the mixture exploded with
increasing ferocity; but pure, 100
percent hydrogen did not ignite.

Cavendish’s thinking was still

handicapped by an obsolete notion

from alchemy that a firelike element

(“phlogiston”) was released during

combustion. However, he was

precise in his experiments and in

his reporting: “it appears that 423

measures of inflammable air are

nearly sufficient to phlogisticate

1,000 of common air; and that the

bulk of the air remaining after

the explosion is then very little

more than four-fifths of the common

air employed. We may conclude

that...almost all the inflammable

air and about one fifth of the

common air...are condensed

into the dew which lines the glass.”

Defining water

Although Cavendish used the term

“phlogisticate,” he managed to

demonstrate that the only new

material produced was water, and

deduced that two volumes of

inflammable air had combined

with one volume of oxygen. In

other words, he showed that the

composition of water is H 2 O.

Although he reported his findings

to Joseph Priestley, Cavendish was

so diffident about publishing the

results that his friend the Scottish

engineer James Watt was the first

to announce the formula, in 1783.

Among his many contributions

to science, Cavendish went on to

calculate the composition of air

as “one part dephlogisticated

air [oxygen], mixed with four of

phlogisticated [nitrogen]”—the

two gases we now know make up

99 percent of Earth’s atmosphere. ■

Henry Cavendish One of the strangest and most

brilliant pioneers of 18th century

chemistry and physics, Henry

Cavendish was born in 1731 in

Nice, France. His grandfathers

were both dukes, and he was

immensely rich. After his studies

at the University of Cambridge,

he lived and worked alone in his

house in London. A man of few

words and shy of women, it was

said that he ordered his meals by

leaving notes for his servants.

Cavendish attended meetings

of the Royal Society for about 40

years, and also assisted Humphry

Davy at the Royal Institution. He

did significant original research

into chemistry and electricity,

accurately described the nature

of heat, and measured Earth’s

density—or, as people said,

“weighed the world.” He died

in 1810. In 1874, the University

of Cambridge named its new

physics laboratory in his honor.

Key works

1766 Three Papers Containing

Experiments on Factitious Air

1784 Experiments on Air

(Philosophical Transactions of

the Royal Society of London)

It appears from these
experiments, that this air, like
other inflammable substances,
cannot burn without the
assistance of common air.
Henry Cavendish