The photo is probably an ambrotype. The presentation of a daguerreotype and an ambrotype was the same -- both were enclosed under glass in a case to protect the fragile photographic emulsion. A daguerreotype will produce silvery mirror-like reflections when viewed at an angle. The ambrotype is a negative image on glass with a dark backing to make it appear as a positive image.
Based on the mat and the clothing, I would date it 1855 to 1860. The ambrotype was not commonly produced as early as 1851. An article on dating ambrotypes is http://www.phototree.com/id_amb.htm