*Effect of Drugs.*—Certain drugs and medicines, including alcohol and quinine,(15) have recently been shown to destroy the white corpuscles. The effect of such substances, if introduced in considerable amount in the body, is to render one less able to withstand attacks of disease. Many patent medicines are widely advertised for purifying the blood. While these may possibly do good in particular cases, the habit of doctoring one’s self with them is open to serious objection. Instead of taking drugs and patent medicines for purifying the blood, one should study to live more hygienically. We may safely rely upon wholesome food, pure water, outdoor exercise and sunlight, plenty of sleep, and a clean skin for keeping the blood in good condition. If these natural remedies fail, a physician should be consulted. *Summary.*—The blood is the carrying or transporting agent of the body. It consists in part of constituents, such as the red corpuscles, that enable it to carry different substances; and in part of the materials that are being carried. The latter, which include food and oxygen for the cells and wastes from the cells, may be classed as freight. Certain constituents in the blood destroy disease germs, and other constituents, by coagulating, close small leaks in the blood vessels. Although subject to rapid and continuous change, the blood is able—by reason of the balancing of materials added to and withdrawn from it—to remain about the same in quantity and composition. Exercises.—1. Compare blood and water with reference to weight, density, color, odor, and complexity of composition. 2. Show by an outline the different constituents of the blood. 3. Compare the red and white corpuscles with reference to size, shape, number, origin, and function. 4. Name some use or purpose for each constituent of the blood. 5. What constituents of the blood may be regarded as freight and what as agents for carrying this freight? 6. After coagulation, what portions of the blood are found in the clot? What portions are found in the serum? 7. What purposes are served by water in the blood? 8. Show how the blood, though constantly changing, is kept about the same in quantity, density, and composition. 9. In the lungs the blood changes from a dark to a bright red color and in the tissues it changes back to dark red. What is the cause of these changes? 10. If the oxygen and hemoglobin formed a strong instead of a weak chemical union, could the hemoglobin then act as an oxygen carrier? Why? 11. What habits of living favor the development of corpuscles in the blood? 12. Why will keeping the skin clean and active improve the quality of one’s blood? PRACTICAL WORK