The phenomenon of coagulation has attracted attention from the earliest times. It was a phenomenon that needed explanation, and culinary experience suggested analogies close at hand. Hippocrates attributed the clotting of blood to its coming to rest and growing cold. The blood which gushed from a warrior’s wound formed a still pool by his side. It set into a jelly as it cooled. Until the second quarter of the nineteenth century this theory was deemed sufficient. It then occurred to two men of inquiring mind to institute control experiments. John Davy placed a dish of blood upon the hob. William Hunter kept one shaking. In both experiments the blood clotted more quickly than it did in vessels of the same size, containing the same amount of the same blood, left upon the table. Even before this date an observation had been made regarding the circumstances in which clotting occurs, which has thrown much light upon the causes of the phenomenon. In 1772 Hewson gently tied a vein in two places. At the end of a couple of hours he opened the vein. The blood was still liquid, but clotted in a normal manner after it was shed. Scudamore showed that blood clots more slowly in a closed than in an open flask. A new theory, as little trustworthy as Hippocrates’, was based upon these observations. Blood clotted because it was exposed to air. A record of all observations of the circumstances of coagulation, and of all the theories to which they have given rise, would make an exceptionally interesting chapter in the history of human thought. It would bring into singular prominence stages in the development of what is now known as the “scientific method.” Not that Science has a method of her own. Philosophers of all classes would follow the same method if their data allowed of its application. The peculiarity of the data with which Science deals is that they can be brought to a test of which the data of historical, or political, or economic theory are not susceptible. They can be confronted with control experiments. The control experiment is the alphabet and the syntax of the scientific method. No hypothesis is admissible into the pyramid of theory until it has passed this test. A natural phenomenon is observed. Every measurement which is applicable is taken and recorded—time, weight, temperature, colour. Scientific observation implies the tabulation of all particulars which are capable of statistical expression. Reflecting upon the relation of the phenomenon to other phenomena of a like nature, the philosopher—it is the philosophy of physiologists which interests us—formulates an hypothesis as to its cause. At this point the real difficulty of applying the scientific method begins. It is easy to formulate hypotheses. It is very difficult to devise control experiments. An experiment must be arranged which will provide that, while all other conditions in which the phenomenon has been observed to occur are reproduced, the condition which was _ex hypothesi_ its