  The novel concept of spectral diffusivity is introduced to analyse the
dissipative properties of continua. The dissipative components of a linear
system of evolution equations are separated into noninteracting parts. This
separation is similar to mode analysis in wave propagation. The new modal
quantities characterise dissipation and best interpreted as effective
diffusivities, or, in case of heat conduction, as effective heat conductivities
of the material.
