Diffusion originally described the way in which heat
(believed to be a fluid) flowed through a solid. Later
the same ideas were applied to describe the way in
which a gas would spread out to fill the available
volume. In solids, diffusion refers to the transport of
atoms, ions or molecules under the influence of a
driving force that is usually a concentration gradient.
Diffusion takes place in solids at a much slower
rate than in gases or liquids, and in the main it is a
high-temperature process. However, this is not
always so, and in some solids the rate of diffusion at
room temperature is considerable.