Brownian movement. Kinetic theory of matter
One day in the year 1827 the botanist, Robert Brown, was using a microscope to examine some pollen particles in water when he was surprised to notice that they were in a continuous state of vigorous haphazard movement. At the time Brown could offer no explanation, but many years afterwards it came to be realized that the motion of the particles is caused by the impact of moving water molecules. The same kind of movement can be seen in the case of smoke particles in air.
Observations of this kind strongly suggest that the molecules of liquids and gases are in a state of continuous motion. This idea of molecular motion and the mathematical calculations which have been applied to it is called the kinetic theory of matter.