The Science Book

139

See also: John Dalton 112–13 ■ James Joule 138 ■

James Clerk Maxwell 180–85 ■ Albert Einstein 214–21

B

y the middle of the

19th century, atoms and

molecules had become

central ideas in chemistry, and

most scientists understood that

they were the key to the identity

and behavior of elements and

compounds. Few thought they

had much relevance to physics,

but in the 1880s, Austrian physicist

Ludwig Boltzmann developed the

kinetic theory of gases, putting

atoms and molecules right at the

heart of physics, too.

In the early 18th century, Swiss

physicist Daniel Bernoulli had

suggested that gases are made of a

multitude of moving molecules. It is

their impact that creates pressure

and their kinetic energy (the energy

of their movement) that creates

heat. In the 1840s and 1850s,

scientists had begun to realize that

the properties of gases reflect the

average movement of the countless

particles. In 1859, James Clerk

Maxwell calculated the speed

of molecules and how far they

traveled before colliding, showing

that temperature is a measure of

the average speed of the molecules.

Centrality of statistics

Boltzmann revealed how important

the statistics are. He showed that the

properties of matter are simply a

combination of the basic laws of

motion and the statistical rules of

probability. Following this principle,

he calculated a number now called

the Boltzmann constant, providing

a formula linking the pressure and

volume of a gas to the number and

energy of its molecules. ■

A CENTURY OF PROGRESS

STATISTICAL

ANALYSIS OF

MOLECULAR

MOVEMENT

LUDWIG BOLTZMANN (1844–1906)

IN CONTEXT

BRANCH

Physics

BEFORE
1738 Daniel Bernoulli
suggests that gases are
made of moving molecules.

1827 Scottish botanist
Robert Brown identifies the
movement of pollen in water,
which becomes known as
Brownian motion.

1845 Scottish physicist John
Waterston describes how
energy among gas molecules
is distributed according to
statistical rules.

1857 James Clerk Maxwell
calculates the mean speed
of molecules and the mean
distance between collisions.

AFTER
1905 Albert Einstein
analyzes Brownian motion
mathematically, showing
how it is the result of the
impact of molecules.

Available energy is the

main object at stake in the

struggle for existence and

the evolution of the world.

Ludwig Boltzmann