This monograph presents results of studies of the physics of blood clotting. The function of surface forces in clotting receives especial attention. For blood transfusion, correct counting of thrombocytes and other purposes, it is desirable to use containers—syringes and vessels—the surfaces of which delay clotting. The author has made an effort to find substances that are more practical than paraffined glass for this purpose. Certain formaldehyde-phenol products delay coagulation, but the most promising substitute for paraffined glass appears to be amber, which, besides presenting surfaces which inhibit clotting for a time, also has the highly desirable quality of transparency. The author believes that his results will lead not only to valuable practical improvements in the handling of fresh blood but also to new and fruitful methods of investigating the blood itself.