Small particles of matter or what we call molecules are in constant motion. This constant motion is called Brownian movement and was proven by experiment. In the year 1872, Robert Brown, an Englishman, put very fine powder in water. The particles moved around as though alive. Brown watched them through his microscope. He found that the movement was caused by water molecules constantly moving and hitting the powder particles. This is what happens to dust in the air when air molecules keep striking the tiny dust particles.
Brown's experiment showed that microscopic particles constantly and randomly vibrate in water or air. In the year 1828, Brown published his paper claiming that unknown forces were active in causing organic objects to move. In 1870s, after Brown's death, a new idea, the Kinetic Molecular Theory, began to gain wide scientific importance. The theory described why moving molecules collide. Only did the deeper significance of Brownian movement come to be known. Brown's unknown forces were clearly explained by this new theory.
Under this theory, all matter is made of invisible particles such as molecules. The atoms or molecules are constantly in motion, in proportion to the temperature and only at absolute zero does all molecular motion stop.
Thus, when microscopically visible large clumps of molecules are floated in air or water, one can see the bigger particles buffeted around by the dashing water molecules, though these latter are invisible. Robert Brown's microscope and his keen eye captured this fact.