The coagulability of the blood is often one of the most important factors with which the clinician must deal. In all cases of bleeding, it is essential to determine whether the cause is a delayed coagulation of the blood or some other factor, such as local infection and injury of blood vessels. This is especially true in purpura, hemophilia, bleeding in the new-born and jaundice, and in preoperative examination of patients. An accurate method of estimating this property of the blood is therefore essential for the proper handling of such cases. For clinical use, the method must be simple and require little apparatus. Many methods have been advocated to determine the clotting time, their very number indicating that none is quite satisfactory. A complete and comprehensive review of all these methods up to 1907 is found in a paper by Hinman and Sladen.1 Since that time, a number of modifications