Ca I absorption and the electron densities in interstellar clouds

New photoelectric detections of interstellar Ca I absorption in 10 stars, combined with new and existing Ca ii data, yield an average Ca I to Ca ii ratio of 6.2 x 10- . It is argued that the Ca I lines and most, if not all, the Ca ii lines are formed in interstellar clouds. For cloud conditions, the Ca I to Ca ii ratio and the assumption of ionization equilibrium give ne 0.09 cm -3, although the average for all clouds might be as small as 0.03 cm -3. Such electron densities require that hydrogen be partially ionized, imply that both Ca and Na are underabundant in interstellar clouds, and suggest that the important cooling elements are also underabundant. Subject headings: abundances, nebular - interstellar matter