Quivering Particles I

Trails of a Wanderer

In 1828, botanist Robert Brown (1773-1858) published a pamphlet titled "A Brief Account of Microscopical Observations on the Particles Contained in the Pollen of Plants."

Brown had traveled widely, exploring the lands around the Cape of Good Hope in Africa and many parts of Australia before returning to London with specimens of more than four thousand plant species for identification. In the course of establishing a major botanical collection at a national museum, he had looked closely at pollen grains obtained from many of the collected plants.

While examining pollen grains of the plant Clarkia pulchella (pinkfairies) under a microscope, botanist Robert Brown observed tiny particles in constant motion within a grain.

Brown noted that, in some cases, "the membrane of the grain of pollen is so transparent that the motion of the…particles within the grain was distinctly visible."

The suspended granules were only a few micrometers in size, less than one-tenth of a typical pollen grain's width. Observed under a microscope, they appeared to be in continuous, erratic motion.

Physician and plant physiologist Jan Ingenhousz (1730-1799), who is credited with discovering photosynthesis, had reported similar particle activity in 1785 when he looked closely at powdered charcoal sprinkled on an alcohol surface.

Brown's experiments with granules extracted from crushed pollen grains, soot particles, and fragments of other materials suspended in water revealed the same type of unceasing, quivering movement.

Some scientists were quick to attribute the effect to minute, heat-induced currents in the liquid surrounding the particles or to some obscure chemical interaction between the solid particles and the liquid. However, the observation that the movements of two neighboring particles appeared to be quite uncorrelated helped rule out currents in the fluid as a cause.

Other characteristics of so-called Brownian motion were just as intriguing. For example, a given particle appeared equally likely to go in any direction, and its past motion seemed to have no bearing on its future course. Particles also traveled faster at higher temperatures, and smaller particles moved more quickly than larger ones. Most striking of all, the jerky movements never stopped.

To explain such observations, some scientists boldly ascribed the phenomenon to molecular movements within the liquid. They pictured the liquid as being composed of tiny molecules whizzing about at high speeds and colliding with each other. When molecules bumped into particles, they would give the particles random pushes.

Although some researchers hailed Brownian motion as visible proof that matter is made up of atoms and molecules, the question of the true structure of matter was at that time still a contentious issue. At the beginning of the twentieth century, for instance, the prominent physical chemist Wilhelm Ostwald (1853-1932) regarded atoms as merely "a hypothetical conception that affords a very convenient picture" of matter.

Next: Quivering Particles II