*To demonstrate the Physical Properties of Blood* (Optional).—Since blood is needed in considerable quantity in the following experiments, it is best obtained from the butcher. To be sure of securing the blood in the manner desired, take to the butcher three good-sized bottles bearing labels as follows: *1* Fill two thirds full. While the blood is cooling, stir rapidly with the hand or a bunch of switches to remove the clot. *2* Fill two thirds full and set aside without shaking or stirring. *3* Fill two thirds full and thoroughly mix with the liquid in the bottle. Label 3 must be pasted on a bottle, having a tight-fitting stopper, which is filled one fifth full of a saturated solution of Epsom salts. The purpose of the salts is to prevent coagulation until the blood is diluted with water as in the experiments which follow. *Experiments.*—1. Let some of the defibrinated blood (bottle 1) flow (not fall) on the surface of water in a glass vessel. Does it remain on the surface or sink to the bottom? What does the experiment show with reference to the relative weight of blood and water? 2. Fill a large test tube or a small bottle one fourth full of the defibrinated blood and thin it by adding an equal amount of water. Then place the hand over the mouth and shake until the blood is thoroughly mixed with the air. Compare with a portion of the blood not mixed with the air, noting any difference in color. What substance in the air has acted on the blood to change its color? 3. Fill three tumblers each two thirds full of water and set them in a warm place. Pour into one of the tumblers, and thoroughly mix with the water, two tablespoonfuls of the blood containing the Epsom salts. After an interval of half an hour add blood to the second tumbler in the same manner, and after another half hour add blood to the third. The water dilutes the salts so that coagulation is no longer prevented. Jar the vessel occasionally as coagulation proceeds; and if the clot is slow in forming, add a trace of some salt of calcium (calcium chloride). After the blood has been added to the last tumbler make a comparative study of all. Note that coagulation begins in all parts of the liquid at the same time and that, as the process goes on, the clot shrinks and is drawn toward the center. 4. Place a clot from one of the tumblers in experiment 3 in a large vessel of water. Thoroughly wash, adding fresh water, until a white, stringy solid remains. This substance is fibrin. 5. Examine the coagulated blood obtained from the butcher (bottle 2). Observe the dark central mass (the clot) surrounded by a clear liquid (the serum). Sketch the vessel and its contents, showing and naming the parts into which the blood separates by coagulation.