Instead of saying, then, that the countries where most great nations have lived are now deserts, we may as well say that most great nations have lived in countries where irrigation was necessary; in fact, that history indicates that a dense population, and high culture, usually go hand in hand with a soil that thirsts for water. What can science, the great explainer, say on this subject? II. "Science moves, but slowly, slowly, moving on from point to point."--_Locksley Hall_. A plant feeds in two ways--by its leaves, and by its roots. The leaves feed from the air; the roots from the soil. In the air is found a colorless, heavy gas, known as carbon dioxide, which is made up partly of the element of carbon, or charcoal. When an animal or a plant is burned at a low heat, it first chars, showing the presence of charcoal; then if the burning be continued, it disappears, with the exception of the ash, as the gas, carbon dioxide. Since animal and vegetable matters are constantly being burned upon the earth's surface, naturally the air contains a perceptible quantity of carbon dioxide. The leaves of a living plant, waving back and forth, draw into themselves the carbon dioxide with which they come into contact, and there break it up and take the carbon away from it. The carbon thus obtained by the leaves is built into the many ingredients of a plant, and carried to the parts that are in greatest need. The plant is able to do this by virtue of the peculiar properties of the green coloring matter in all its leaves, leaf green; which acts, however, only in the presence of bright sunlight. Since one-half or more of the dry matter of a plant is carbon, the importance of the leaf-air-feeding of a plant may be understood. The water which a plant contains and the incombustible portions, the mineral matters or ash, are taken directly from the soil by means of the roots. The old idea that vegetable mould and other corbonaceous matters are also taken from the soil by the roots has been shown to be erroneous. The mineral portions of a plant are of the highest value to the life of the plant--without them, in fact, it languishes and dies. If a soil on which a plant is growing contains, for instance, no iron, the leaves become pale, soon white, and finally they lose the power of appropriating carbon from the air. If potash is absent from the soil, the plants growing upon it will develop in an imperfect manner and finally die. It has been found by careful experiment that seven mineral substances must be found in every soil, if it shall support the life of plants, namely: (1) Potash; (2) lime; (3) magnesia; (4) oxide of iron or iron rust; (5) sulphuric acid or oil of vitriol; (6, phosophoric acid, and (7) nitric acid or aqua fortis. The fertility of any soil or soil district is determined by the quantity of these indispensable ash ingredients contained by it.