Q:  What is the volume of this cylinder, in cubic kpc?

           V  =  (pi * R^2) * h   =  (pi * (20000 pc)^2 * 3000 pc)  

              =  3.8 x 10^(12) cubic pc


  Q:  What is the volume of this cylinder, in cubic cm?

           1 pc  =  3.08 x 10^(16) m  =  3.08 x 10^(18) cm

           1 cubic pc  =  [ 3.08 x 10^(18) cm ]^3  =   

                        =   2.92 x 10^(55) cubic cm


           volume of disk  =  3.8 x 10^(12) pc^3 * 2.92 x 10^(55) cm^3/kpc^3

                           =  1.1 x 10^(68) cm^3


  Q:  Assuming that the hot ionized phase completely fills this
        cylinder (which it does not), how much mass would it contain?

          number of atoms  =  1.12 x 10^(68) cm^3 * 0.0065 atoms/cm^3

                           =  7.2 x 10^(65)  atoms


       If we assume all the atoms are hydrogen,

          mass of atoms    =  7.2 x 10^(65) * 1.67 x 10^(-27) kg

                           =  1.2 x 10^(39) kg

                           =  6.1 x 10^8    solar masses


  Q:  How does that compare to the measured masses of the other
        phases of the ISM?


            This is smaller than the mass of molecular gas,
               and only about one-tenth the mass of atomic gas.