A Herschel [C ii] Galactic plane survey - II. CO-dark H2 in clouds W. D. Langer, T. Velusamy, J. L. Pineda, K. Willacy and P. F. Goldsmith A&A, 561 (2014) A122 Published online: 21 January 2014 DOI: 10.1051/0004-6361/201322406