While the air in passing in and out of the lungs is thus robbed of a portion of its oxygen, and loaded with a certain quantity of carbonic acid, the blood as it streams along the pulmonary capillaries undergoes important correlative changes. As it leaves the right ventricle it is venous blood of a dark purple or maroon color; when the blood has passed through the lungs and falls into the left auricle, it is arterial blood of a bright scarlet hue. In passing through the capillaries of the body from the left to the right side of the heart, it is again changed from the arterial to the venous condition. The average composition of this gas in the two kinds of blood is as follows. From 100 volumes may be obtained: Oxygen. Carbonic Acid. Nitrogen. Of arterial blood, 20 (16) vol. 39 (30) vol. 1 to 2 vol. Of venous blood, 8-12 (6 to 10) 46 (35) vol. 1 to 2 vol. Oxygen plays a most important role on this terrestrial globe. Life, health, and food depend on it. This element penetrates, pervades, everything and everywhere, unites and disunites with all other elements, preserves and destroys. While its absence from a living being, whether plant or animal, is death. When a liquid such as water is exposed to an atmosphere containing a gas such as oxygen, some of the oxygen will be dissolved in the water, that is to say will be absorbed from the atmosphere. The quantity which is so absorbed will depend on the quantity of oxygen which is in the atmosphere above; that is to say, on the pressure of the oxygen; the greater the pressure of the oxygen, the larger the amount which will be absorbed. If, on the other hand, water containing a good deal of oxygen dissolved in it be exposed to an atmosphere containing little or no oxygen, the oxygen will escape from the water into the atmosphere. CHAPTER XX. DIGESTION, NUTRITION. In plant life the permanent fabric consists of only three elements--carbon, hydrogen, oxygen. We know that plants alone convert inorganic or mineral substances into organic matter, and that plants as a necessary result assimilate their inorganic food, decompose carbonic acid, and restore its oxygen to the atmosphere.