Sound

Movement of Pollen Grains in Water

The best evidence for the existence and movement of particles in liquids was given by Robert Brown 1827. Robert Brown suspended extremely small pollen grains in water. On looking through the microscope it was found that the pollen grains were moving rapidly throughout water in a very irregular way (or zig-zag way) (see Figure 6). It was also observed that warmer the water, faster the pollen grains move on the surface of water.
The movement of pollen grains on the surface of water can be explained as follows : Water is made up of tiny particles which are moving very fast (The water particles or water molecules themselves are invisible under the microscope because they are very, very small). The pollen grains move on the surface of water because they are constantly being hit by the fast moving particles of water.So, though the water particles (or water molecules) are too small to be seen, but their effect on the pollen grains can be seen clearly. The random motion of visible particles (pollen grains) caused by the much smaller invisible particles of water is an example of Brownian motion (after the name of the scientist Robert Brown who first observed this phenomenon).
Brownian motion can also be observed in gases. Sometimes, when a beam of sunlight enters a room, we can see tiny dust particles suspended in air which are moving rapidly in a very haphazard way. This is an example of Brownian motion in gases (because air is a gas). The tiny dust particles move here and there because they are constantly hit by the fast moving particles of air.Though we cannot see the particles (or molecules) of air which are extremely small, but we can see the effect produced by their continuous and fast motion. The rapid and random movement of tiny dust particles (or smoke) suspended in air shows that air is made up of particles and that the particles of air are moving constantly. We can now define Brownian motion as follows. The zig-zag movement of the small particles suspended in a liquid (or gas) is called Brownian motion. Brownian motion increases on increasing the temperature.
The existence of Brownian motion gives us two conclusions about the nature of matter:
(i) that matter is made up of tiny particles, and
(if) that the particles of matter are constantly moving.