492. As the impure blood is passing in the minute vessels over the air-cells, the oxygen passes through the thin coats of the air-cells and blood-vessels, and unites with the blood. At the same time, the carbonic acid and water leave the blood, and pass through the coats of the blood-vessels and air-cells, and mix with the air in the cells. These are thrown out of the system every time we breathe. This interchange of products produces the change in the color of the blood. -=-=-=-=-=-=-=-=-=-=-=-= Explain fig. 98. 492. How and where is the blood changed? -=-=-=-=-=-=-=-=-=-=-=-= _Experiment._ Fill a bladder with dark blood drawn from any animal. Tie the bladder closely, and suspend it in the air. In a few hours, the blood next to the membrane will have become of a bright red color. This is owing to the oxygen from the air passing through the bladder, and uniting with the blood, while the carbonic acid has escaped through the membrane. [Illustration: Fig. 99. An ideal view of the pulmonary circulation. 1, 1, The right lung. 2, 2, The left lung. 3, The trachea. 4, The right bronchial tube. 5, The left bronchial tube. 6, 6, 6, 6, Air-cells. 7, The right auricle. 8, The right ventricle. 9, The tricuspid valves. 10, The pulmonary artery. 11, The branch to the right lung. 12, The branch to the left lung. 13, The right pulmonary vein. 14, The left pulmonary vein. 15, The left auricle. 16, The left ventricle. 17, The mitral valves.] 493. The presence of carbonic acid and watery vapor in the expired air, can be proved by the following experiments: 1st. Breathe into lime-water, and in a few minutes it will become of a milk-white color. This is owing to the carbonic acid of the breath uniting with the lime, forming the _carbonate of lime_. 2d. Breathe upon a cold, dry mirror for a few minutes, and it will be covered with moisture. This is condensed vapor from the lungs. In warm weather, this watery vapor is invisible in the expired air, but in a cold, dry morning in winter, the successive jets of vapor issuing from the mouth and nose are sufficiently obvious. -=-=-=-=-=-=-=-=-=-=-=-= Give the experiment showing that oxygen changes the dark-colored blood to a bright red color. What is represented by fig. 99? 493. How can the presence of carbonic acid in the lungs be proved? -=-=-=-=-=-=-=-=-=-=-=-= 494. From the lungs are eliminated other impurities beside carbonic acid, the perceptible quality of which is various in different persons. The offensive breath of many persons may be caused by decayed teeth, or the particles of food that may be retained between them, but it often proceeds from the secretion, in the lungs, of certain substances which previously existed in the system.