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 [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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