The vapours rising from sea and land are condensed into rain on meeting with currents of cold air, or impinging on mountain-tops. When first formed the rain differs little, if at all, from distilled water, but in its fall it takes to itself various gases and suspended matters from the air. On reaching the earth, part is evaporated and part absorbed, the proportions varying with the perviousness of the soil, and with other conditions; if the rainfall be heavy and the face of the land inclined, and especially if the soil be rocky or plastic, much runs off as storm-water to the nearest stream or lake. Of that which is absorbed, part is taken up by the roots of trees and plants, to be again evaporated from their leaves; but the largest portion sinks downwards until, meeting with some impervious stratum, it forms a sheet of water, filling the interstices between the particles of the soil. This is the "ground-water", to which reference has already been made. In some localities it is nearly stagnant, riling and lulling with rain and drought; hut usually it flows slowly onwards, following the decline of the impervious stratum which forms its bed, - and which, by the way, need not correspond with the surface-contour, - until it finds an exit as a spring or enters the channel of a river. During its course through the soil it takes up everything organic, inorganic, or gaseous, capable of being dissolved.

The mineral constituents of all waters take their character from the nature of the beds through which they have percolated. The most important are the carbonates, chlorides, and sulphates of potassium, sodium, calcium and magnesium among the alkalies and alkaline earths, and of iron among the metals. Magnesium and iron are found only in certain localities, and iodides, bromides, lead, arsenic, etc, still more rarely. These, as well as the chlorides, and sulphates of sodium and magnesium, when present in amounts rendering the water unfit for ordinary household use. constitute it a medicinal water. Calcium and magnesium salts give hardness, and an excess of chlorides, as in salt water, has a similar effect.

"Aeration" is an equivocal expression. Water exposed to the air absorbs its constituent gases, though in different proportions oxygen being far more soluble than nitrogen. Water in the soil absorbs the ground-air, in which carbonic dioxide (CO2) is present in proportions enormously in excess of that found in the atmosphere. The so-called aeration of spring-water is therefore very different from that of river-water: the "air" contained in the latter is composed largely of oxygen, and will support the respiration of fishes, but that in spring-water consists mainly of carbonic dioxide, which is irrespirable. In the small quantity in which the latter usually occurs, it is the product to a great extent of the decomposition of organic matter, but the carl ionic dioxide in such mineral waters as Seltzer, Johannis, etc, has a different origin. However derived, it imparts to the water the power of dissolving carbonate of calcium, or chalk, by which the water is rendered "hard ".

Hard waters are incapable of producing a lather with soap, until the whole of the lime present has been used up in combination with the fatty acids of the soap. The effect of boiling hard water is, by expelling the free CO2 to throw down the carbonates of lime, and of magnesia should any be present, rendering the water soft, but depositing in the kettle or boiler, as well as in hot-water pipes, a crust composed of impure earthy carbonates, which may easily be dissolved by a weak acid, as vinegar or dilute hydrochloric acid. So much of the hardness as is due to the presence of calcium and magnesium carbonates, and is removable by boiling, is called "temporary hardness", but since other salts, as chlorides and sulphates, cannot be thus precipitated, the hardness they cause is called "permanent". Very hard waters, as those from the chalk, are softened on a large scale by adding so much "milk of lime" as shall, by combining with the free CO2, be converted into carbonate, when the chalk previously in solution, together with that thus produced, falls as a sediment, and the clear and now much-softened water is pumped or siphoned off.

All water that has percolated through the soil contains chlorides, chiefly sodic chloride or common salt. The quantity in lakes and rivers is generally less than in springs and wells, since the former are more or less fed by storm-waters, and rain that has not penetrated the soil.

In the economy of nature all organic matters, carcasses, excreta, etc, soaking into, or buried in the upper soil, undergo decomposition through the action of bacteria. These swarm in the superficial layers to the depth of a few feet, but are not found at greater depths. They are absent from pure sand, chalk, rocks. and plastic clay, but appear in these so soon as, by digging and the admixture of other earths and manures, they are converted into "soil". Thus, sewage-farms in poor sandy districts, as those of Berlin, are rendered more efficient by the constant incorporation of the sludge. Light friable sandy loams are by far the best purifiers; but even in them the "top spit" is the most active, the number of bacteria rapidly decreasing downwards, until below six or eight feet they have practically disappeared. This upper zone has been well described by Dr. Poore as "the living earth", all below being "dead", or devoid of active living organisms. Putrefaction is effected by other bacteria, and is attended by the evolution of stinking gases (ammoniacal compounds with sulphur and phosphorus), but in the decomposition under consideration the agents are the so-called "nitrifying bacteria", by which the albuminous matter and these ammoniacal products are further oxidized into nitrous and nitric acids, which combine with the earthy bases in the soil to form nitrates, and in that form furnish the principal food of plants; this process, in fact, is that on which the whole theory of manures is based, the conversion of animal substances into mineral salts, which alone plants can absorb and transform into vegetable albumen, etc.