Diffusion

Diffusion, in Physics, is that property possessed by the particles of liquids and gases, of travelling from place to place by reason of the energy with which they are endowed. In a boiler, where steam at a high temperature is enclosed in contact with water at the same temperature, the particles are endowed with sufficient energy to pass incessantly from any one part of the enclosure to any other, whether from the liquid to the gas or back to the liquid again. In a glass of water that is apparently motionless the particles are continually, though slowly, moving from place to place in the body of the liquid; and though opposed in part by the skin-like action of the surface of the water, they continually effect a passage into the surrounding air and probably an occasional return to the liquid. The fact that diffusion actually occurs is more readily shown by the use of two liquids of different colours. Graham poured a strong solution of bichromate of potash along the bottom of a glass cylinder half filled with water, so carefully as to avoid mixing the liquids. Though the lower had the greater density, it was found that the sharp line of separation of colourless water from the deep red potash salt solution gradually became indistinct, and traces of colour appeared higher and higher in the cylinder of water until uniformity of tone and constitution was reached. This proved that particles of the heavier liquid had in opposition to gravity risen into the lighter medium above, and that the lighter water particles had similarly descended.