<P> Convection is the heat transfer due to bulk movement of molecules within fluids such as gases and liquids, including molten rock (rheid). Convection takes place through advection, diffusion or both . </P> <P> Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place . Diffusion of heat takes place in rigid solids, but that is called heat conduction . Convection, however, can take place in soft solids or mixtures where solid particles can move past each other . </P> <P> Thermal convection can be demonstrated by placing a heat source (e.g. a Bunsen burner) at the side of a glass full of a liquid, and observing the changes in temperature in the glass caused by the warmer ghost fluid circulating into cooler areas . </P>

In which state of matter does convection not happen