  Observationally, both the 3.4micron aliphatic hydrocarbon C--H stretching
absorption feature and the 9.7micron amorphous silicate Si--O stretching
absorption feature show considerable variations from the local diffuse
interstellar medium (ISM) to Galactic center (GC): both the ratio of the visual
extinction (A_V) to the 9.7micron Si--O optical depth (\tausil) and the ratio
of A_V to the 3.4micron C--H optical depth (\tauahc) of the solar neighborhood
local diffuse ISM are about twice as much as that of the GC. In this work, we
try to explain these variations in terms of a porous dust model consisting of a
mixture of amorphous silicate, carbonaceous organic refractory dust (as well as
water ice for the GC dust).
