Copyright (C) Michael Fowler 1998 under Gnu Public License

Brownian Motion

On the right – after pressing the start button –, the jiggly movement of a tiny particle observed through a microscope (use checkbox below to see the particle's trace).

On the left – after the appropriate checkbox below has been checked –, the Daltonian explanation: buffeting by (much tinier) molecules.

Please press the Start button and adjust the number of small particles according to your screen size.

More on what Brown saw ... and how Einstein used it to find molecular size

Biologist Robert Brown, studying the sex life of plants, noticed in 1827 that very tiny (a few microns size) granules from inside pollen grains could be seen jiggling around under the microscope. At first he thought this was a sign of life, but then checked with equally tiny particles of rock, and saw the very same motion. After a few decades, it was realized that this must be caused by molecules bouncing off the particle at random. Then Einstein figured out that the smaller the molecules (and therefore more of them for a given density liquid) the less the particle would be jiggling, and with some clever math, Einstein was able to find the size of the molecules from measurements of the particle's path--one of the first reliable measurements of molecular size. All the details are in my lecture here.