Let us find the antipodes of weight. Iridium, hammered to increase its density, is twenty-three times heavier than water; water is about eight hundred times heavier than air, and air is fourteen times heavier than H: 23×800×14=257,600; that is, one quart of Ir would balance 257,600 quarts of H. OXYGEN. There are four kings among chemical substances: Oxygen, king of all the elements; gold, king of the metals; oil of vitriol, king of the acids, and potash, king of the bases. [Illustration: PREPARATION OF OXYGEN FROM MERCURIC OXIDE—MATERIALS USED BY DR. PRIESTLY.] This term of distinction is given to oxygen because of its marvellous activity and range of powers; it unites with all elements save one, fluorine. Its grasping disposition is often resisted by man; he keeps it from destroying his house by painting it; from gnawing at the quivering nerves of his teeth by filling them; from devouring his fruits by canning them; and Monsieur Goffart has now taught us to save green food for our cattle, from its ravages by excluding O from our silos. In spite of its destroying power we can not live without it. The light and warmth in our homes are produced by its rapid union with fuel. Every moment we breathe we are absorbing it into our bodies, where it unites with waste matter, producing heat and energy, and removing that which would clog and poison the system. There is nothing in nature more beautiful than the plan by which the animal and vegetable kingdoms mutually sustain each other by the interchange of O. Look at this little aquarium; here are two or three shiners, some goldfish, and a few water plants. In this little world we may see exactly what goes on in the great world. That goldfish is inhaling O, which is conveyed into the capillaries, unites there with the carbon, forming CO₂, which is exhaled, seized upon by the plant, and in the wonderful laboratory of its cells, the C is separated from the poisonous gas, and retained, while the O is thrown off, again to be used by the fish. Upon the nice adjustment of the plants to the animals, and vice versa, depends the life of both. While upon this subject we might note another interesting evidence of beneficent design in the provision made for both fish and plants. Water absorbs gases with great readiness—some of them it takes more readily than others; for example, a pint of water will absorb seven hundred pints of ammonia gas. It will take but its own volume of carbonic anhydride under one pressure of the atmosphere.