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"... The physical theory of Brownian motion ended the skepticism about the existence of atoms and molecules as actual physical entities." [1]

Brownschen Bewegung


It is interesting that the Scottish botanist Robert Brown, one of the first (1827) to study the fluctuations now known as Brownian motion, initially thought that microscopic molecules were animated; a view he quickly retracted [2].

The modern view of Brownian motion was first expressed by A. Einstein (1905), based on the kinetic theory of heat (that heat was due to the kinetic energy of atoms).

Einstein wrote later that his major aim was to find facts that would guarantee as much as possible the existence of atoms of definite size [1].

[1] Brownian Motion, Encyclopedia Britannica.
[2] Remarks On Active Molecules, Robert Brown, 1829, John Park's ChemTeam: Classic Papers from the History of Chemistry (Cached)

On The Web

Brownian motion should not be too hard for an amateur to observe. The following site has useful suggestions.
Seeing Brownian Motion, microscopy-uk, 2001 (Cached)