On ionization in the H I regions

A comparison is made of the K I, Na I, Ca ii, and H I interstellar absorption lines formed in relatively low-density interstellar clouds in H I regions. The data indicate that the fractional ionization fl,/flH is nearly constant for a range in densities nH covering a factor of 15. If ionization balance exists in the clouds, then ionization of hydrogen by either low-energy cosmic soft X-rays cannot he the principal source of the observed free electrons, and the hydrogen ionization rate is less than t(h16 l for these processes. Subject headings: cosmic rays - interstellar matter