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Dalton in 1801 filled two bottles,
 | Fig: Gaseous diffusion Daltons experiment on gaseous diffusion. |
one with hydrogen and the other with carbon dioxide, and connected them by a long vertical glass tube, the light gas being above and the heavy gas below. After several hours the gases were uniformly mixed, as may be shown by opening each under caustic soda solution and measuring the absorption. This spontaneous mixing of gases in opposition to the force of gravity is called diffusion, and must be due to the motion of the molecules of the gases amongst each other. This motion is not perceptible to the eye because the molecules are very minute.
Similar diffusive motions occur in liquids, but even more slowly. If a tall cylinder is filled with water, and a layer of copper sulphate crystals placed at the bottom,
 | Fig: Liquid diffusion |
the salt dissolves, and a layer of blue solution is formed. If the jar is set aside in a room of uniform temperature, to avoid convection currents, the blue colour slowly rises through the jar until, after several months, the colour of the solution has become uniform.
We therefore assume that the molecules of liquids and gases are in ceaseless motion, in much the same way as a swarm of gnats on summer evening. This picture of the condition of a molecular swarm, as conceived to exist in gases and liquids, is called the kinetic theory (Greek kinesis, motion).
From the slowness of diffusive motion it might seem that the molecular speeds must be small. This is not correct; the molecules in air, for example, move with speeds of the order of a quarter of a mile per second In the same way the gnats in the swarm are moving about with considerable speeds, although the swarm itself is nearly stationary.
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