Q:  If the hot phase of the ISM has a density of n(H) = 0.001 atom/cm^3,
            then we find mean free paths

               lambda = 912 Angstroms


                    sigma  approx     7 x 10^(-18)  cm^2

                    L      approx   1.4 x 10^(20)   cm  =   500 pc



               lambda = 100 Angstroms     (UV/X-ray boundary)

                    sigma  approx     2 x 10^(-20)  cm^2

                    L      approx     5 x 10^(22)   cm  =   16,000 pc

                          This should penetrate most of the Milky Way


               lambda =  10 Angstroms     (soft X-rays)

                    sigma  approx     1 x 10^(-22)  cm^2

                    L      approx     1 x 10^(25)   cm  = 3,200,000 pc

                          This should also penetrate most of the Milky Way