The Nature of Hæmatocolpos Fluid, with Deductions as to the Chemical Composition of the Menstrual Discharge and the Cause of its Normal Non-Coagulability, Also of the Acidity of the Vaginal Secretion; and the Character of the Obstructing Membrane, with its Bearing on the Development of the Vagina; Together with a Report of Three Cases of Hæmatocolpos