The descending rain drops absorb these two gases and convey them to the rootlets of the plant, for food. More wonderful still, the Almighty has arranged that water should remove O from the air more readily than it does nitrogen; consequently the rain carries down the O to the fish in river, lake and ocean, adding its life-giving principle to the air, which is always contained in water. It is a pretty sight to watch the breathing of a fish as he sends the rapid currents of water through his gills in the act of aërating the blood, which, as it passes through, gives them a crimson color. It may easily be proved that plants throw off O, by submerging any vigorous growing plant in a jar of water; in a short time little bubbles will be seen clinging to the leaves; now fill a bottle with water, invert, and touch the little globules gently, when they will detach themselves and pass up into the bottle, displacing the water, and may afterward be used in experimentation. Perhaps some of you, while drinking at the brook, have noted these bubbles of O on the leaves of the graceful water plants below. This is the only place in nature where you can see O free, and indeed you do not see it here, for O is a colorless, tasteless, odorless substance; what you do see is the thin sphere of water which contains it. O is held by many substances so tenaciously that we can not liberate it; this gives us “terra firma.” Sand, and many rocks consist of O and silicon, but the greatest heat and heaviest blows can not separate them. There are materials, however, which readily yield their O. Dr. Priestly first found it by heating with a burning glass a compound known now as red oxide of mercury. The O went off, leaving the shining quicksilver. You may repeat this historic experiment by placing the material in a test tube and heating it over an alcohol lamp. Another substance used for this purpose is black oxide of manganese (MnO₂), but that which is now generally employed is a white salt, kept by every druggist, and usually called chlorate of potash (HClO₃). [Illustration: PREPARATION OF OXYGEN FROM A MIXTURE OF POTASSIC CHLORATE (CHLORATE OF POTASH) AND MANGANESE DI-OXIDE.]