The small angle scattering of radiation or particles from many porous
solids may be described in terms of fractal geometry. To date most exa
mples have been surface fractals but a few studies have suggested that
some porous solids may be mass fractals. We give a derivation of the
scattering law from such a solid and present an experimental example o
f such scattering behavior. (C) 1996 American Institute of Physics.