The idea that everything is made up of very small particles has been developed over many years supported by several experiments:

(i)               Brownian motion: Robert Brown was an English botanist who, in the year 1827, observed pollen grains floating on water under a microscope. He observed that pollen grains moved arbitrarily over the surface of water.

This arbitrary motion of pollen grains was named Brownian motion and later explained to be caused because of collision of the water molecules with the pollen grains. The water molecules are too small to be seen with any microscope, but it is evident from movement of pollen grains that the water molecules exist and are in constant random motion.

(ii)              

Dilution of colour: if we add a drop of ink to the beaker full of water it will be observed that the ink spreads throughout the water until all the water is coloured. This is because the ink particles take up the spaces inbetween the water molecules and, as they are in constant random motion, the ink particles spreads evenly throughout the water. 

(iii)               Diffusion of gases: gases mix with each other by diffusion (process by which the solute moves from the region of high concentration to the region of low concentration). The common examples include the spread of a fragrance from 1 end of a room to another or from 1 room to another.

                Diffusion of bromine from 1 gas jar to the other

Experimentally diffusion can be checked by the reaction of hydrogen chloride gas with ammonia. 2 cotton balls, 1 soaked in hydrochloric acid and another in the ammonia solution, are placed at opposite ends of a glass tube and ends are sealed.

After some time the white fumes of ammonium chloride forms where the 2 gases mix. The fume is seen nearer the hydrochloric acid, as hydrochloric acid, as it is heavier, diffuses slower than ammonia.