Motion of Molecules (English)

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1-4: The Motion of Molecules

[Doubt about Atomism]

The kinetic theory of gases

and

statistical mechanics

were developed by

J. C. Maxwell

(UK, 1831 - 79)

and

L. Boltzmann

(Germany, 1844 - 1906)

in the 19th century.

These theories

are based

on the theory of atoms

(atomism)

and molecules.

Looking at the success

of these theories,

they thought that the theories of

the atomism and

of molecules

were firmly established.

However, even

at the beginning of

the 20th century,

some leading physicists

strongly opposed

to the ideas of

atomic

and molecular theory,

insisting that

the idea is nothing

but a product

of human imagination

and

there had been no

experimental proof

of the existence

of atoms and molecules,

although the idea

was very convenient.

What smashed

this opposition

was the observation

of Brownian motion.

[Speed of the Molecular Motion]

Molecules in a gas

are always moving

rapidly and busily.

J. P. Joule

(UK, 1818 - 89)

estimated the speed

of a molecule in a gas

for the first time

(1851).

He thought that

a gas molecule

in a container

moves around

at a high speed

and sometimes collides

against the wall

of the container.

When colliding,

the molecule exerts

forces on the wall.

The forces are summed up

as the pressure

of the gas.

This is Joule's

basic idea

on gaseous pressure

from the viewpoint

of the molecular theory.

Standing on this viewpoint,

let us estimate

the molecular speed

in a gas below.

To do this,

we need some mathematical

expressions.

Although they are not

so difficult,

let us explain

on the other page:

1-4-A: Speed of Gaseous Molecules

As seen on the page

(1-4-A),

a molecule in a gas

is moving at an extremely

high speed,

almost equal

to the speed of sound.

It is generally

thought that

molecules in a gas

and a liquid

are moving randomly

at a very high speed.

We cannot watch

those motions directly,

but, if we can observe

them in any indirect way,

we could confirm

the existence of molecule.

This has been enabled

by the observation of

Brownian motion.

[Brownian Motion]

A botanist

R. Brown

(UK, 1773 - 1858)

observed irregular

motions of very fine

particles

which came out of pollens

floated on water.

He put pollens

on water,

where they absorbed

sufficient water

to be burst into

very fine particles.

He observed the irregular

zigzag motions

of these particles

using a microscope

(1827).

He reported that

such fine particles

come not only out of

pollens but also

from other substances

and they show similar

irregular motions.

This irregular motion

was called

Brownian motion

that is thought

to be a rather

universal motion

seen not only in such

a liquid as water

but also for a particle

of soot in air.

It appears to occur

because

the fine particle

is violently colliding

with molecules of the gas

or the liquid

moving randomly

at high speed.

A. Einstein

(Germany, USA, 1879 - 1955)

gave a theoretical

interpretation

of Brownian motion

based on the viewpoint

of the molecular theory

(1905).

In 1908 - 13,

J. B. Perrin

(France, 1870 - 1942)

carried out

a lot of difficult

experiments repeatedly,

and he was finally

successful in proving

Einstein's

molecular theory.

Thereby he succeeded

in estimating

the size of the molecule

and obtaining the value

of Avogadro's constant.

Looking at the results

of Perrin,

the actual existence

of atoms

and molecules

has widely been

recognized and accepted.

An Example of Brownian Motion observed by Perrin.

The position

of a fine particle

of olibanum

is recorded

for every constant

interval.

The actual motion

is three-dimensional,

but the figure shows

the position

projected on a plane.

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