density    n   ~    25   atoms  per cubic meter

    diameter   d   ~     1   Angstrom  =  10<sup>-10</sub> meters


    Q:  What is the mean freee path of a hydrogen atom in an HI cloud?



                                             1
                           L   =      --------------
                                       pi * d^2 * n



                                              1
                               =      ----------------------------------
                                     pi * [ 10^(-10) ]^2 * 25,000,000


                               =        1 x 10^(12)  m    =   7 AU