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Brownian Motion

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The term 'Brownian motion' (or 'Brownian movement') refers to the apparently random, haphazard movement of microscopic particles which are suspended in a fluid.

Although a number of earlier workers had observed this phenomenon, it was first described, and therefore named after, the British botanist, Robert Brown, who was studying pollen grains in 1827. Brown was an accomplished microscopist. It was he who, for example, first identified the naked ovule in the gymnospermae1; this is a difficult observation to make even with a modern instrument.

Brown was attempting to further his work on the mechanisms of fertilisation in flowering plants (which he had published early in 1826) and having worked on the ovum, he now wished to direct his attention to the structure of pollen and its interrelationship with the pistil2. The first species to which he turned his attention in June 1827 was the American species, Clarkia pulchella3. He believed that he would be able to examine the pollen grains more effectively through his microscope if they were suspended in water, a technique known as 'water-immersion'. To his annoyance, he observed that the pollen grains danced continuously and erratically around in the water, thus interfering with his observations. From these observations he satisfied himself that the movement:

'arose neither from currents in the fluid, nor from its gradual evaporation, but belonged to the particle itself'.

He first ascribed this to some manifestation of the 'vital force' which many people at that time still believed all organic matter to be imbued with. Having observed this phenomenon in a range of living specimens, he did the 'control' experiment and was able to observe exactly the same behaviour in pollen grains which had been preserved for eleven months in alcohol solution - ie, they were dead. Having realised that the phenomenon was also exhibited by non-living matter, Brown went on to examine other non-living materials, such as finely-ground rock.

Although Brown's observation of pollen grains4 in water was the first recorded example of this phenomenon, it is also possible to observe the phenomenon in air using smoke particles. Here, a 'smoke cell,' which can be bought commercially, is used and which consists of a cell into which smoke from smouldering paper or string may be introduced using a teat pipette. The cell is illuminated from the side using light from a small bulb. There is a light disperser between the bulb and the cell. Once the smoke has been introduced, a cover slip is applied to the cell, and the contents viewed from above through a microscope under low power. The smoke particles are visible as bright pin-pricks of light under constant erratic movement.

This experiment is cited as strong evidence for the constant movement of air particles which, of course, are invisible - although their presence can be felt when there is a draft or a breeze. Brownian motion in general is also cited as strong evidence for the continuous motion of particles in liquids and gases. It is further realised that the atoms which make up the solid state of matter are also continually vibrating about a mean position. This idea of atomic/molecular motion and the associated mathematical calculations is known as the kinetic theory of matter.

During the early 20th Century, Einstein and the French scientist JB Perrin were able to show that Brownian motion provided direct evidence of the existence of atoms and molecules. This is discussed in more detail in the Guide Entry Atoms and Atomic Structure .

Despite all of this knowledge, scientists continue to be fascinated by the origin and nature of Brownian motion, which is still imperfectly understood. Articles concerning the mathematics of Brownian motion continue to be published in contemporary physics journals.

1Group of plants which includes the conifers and their allies.2Female part of the flower.3A plant of the Evening Primrose family.4Other particles which may be used to visualise Brownian motion in water include finely divided graphite and lycopodium powder. Lycopodium powder consists of the spores from a group of plants known as club mosses. Exposure to these can cause allergic reactions.

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