In 1827, Robert Brown, a Scottish botanist, prepared a slide by adding a drop of water to pollen grains. As he watched the tiny particles of pollen under his microscope, Brown noticed that they were constantly jiggling about. He thought that the motion might be related to life processes within the pollen, but later he observed the same kind of zigzag motion with pollen from plants that had been dead for many years. Others found the same strange motion when they observed tiny inanimate particles of dye, dust, smoke, or soot.
Brown could offer no explanation for his observation, which became known as Brownian motion, nor could anyone else until James Clerk Maxwell and others developed the kinetic molecular theory a generation later. According to this theory, Brownian motion was the result of collisions between the small microscopic particles and the invisible but constantly moving water or air molecules surrounding them. Since particles such as pollen are thousands of times larger than water molecules, we would expect that on the average the particle would be hit as many times by water molecules on one side as it would be by molecules striking it from the opposite direction. However, since molecular motion is random, there will be moments when the particle is struck by more molecules moving in one direction than in any other. When that happens, the particle will respond to the unbalanced force and move accordingly.
Imagine yourself caught in the middle of a large crowd of people who are undecided as to which way they should go. You would find people bumping into you from all sides. Sometimes pushes from all directions would be equal and you would not move. At other times, more people would be bumping you from the right than from the left and so you would move to the left. A short time later, there might be more people pushing from behind than from in front, and you would move forward. Your motion would be similar to that of a tiny pollen particle suspended in, and constantly being struck by randomly moving molecules of water.
The fact that the jiggling movement of a particle exhibiting Brownian motion increases with temperature provided evidence that its motion could be explained by the kinetic molecular theory. Early in the twentieth century, Albert Einstein published a series of papers in which he statistically analyzed the expected velocity of particles of various sizes and masses undergoing Brownian motion at various temperatures in liquids with different viscosities. In an effort to verify Einstein's theoretical work, Jean Perrin carried out a number of experiments using small uniform particles of known size and mass. His results confirmed Einstein's analysis and put to rest forever any doubts about the molecular nature of matter.
See also Molecule.
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