, The chief points of interest on this subject may be arranged under the heads of the sources of supply, the qualities of water, and the arrangements for its conveyance and distribution.
Sources of 1Vater.
The ultimate source of all fresh water is rain (q.v.). When it has fallen on the earth, it presents itself chiefly in the forms of surface-water, rivers, and springs.
Surface-collectkn. —Rain-water, as it is formed in the upper regions of the atmosphere, is the purest that nature supplies; but in descending, it brings with it whatever impuri ties are floating near the surface, which, in the neighborhood of towns, are numerous, consisting of various gases, together with soot and other floating particles, organic and inorganic. Rain-water has a strollg affinity for organic inpurities—that is, the corrupt ing ingredients derived from vegetable and animal bodies, and which are diffused over every surface in the vicinity of living beings; hence, when collected from the roofs of houses, it has a tendency to rapid putrefaction. Being free from saline ingredients, it is excellent for washing, but is not generally pleasant to drink.
But if we resort to a barren district of rock, destitute of vegetation, and remote from the pollution of towns, we may obtain water with comparatively little organic impurity. Notwithstanding several defects, it happens in various places that a surface-supply is the best that can be had.
.Rivers.—The water obtained from running streams is in part what has flowed immediately from the surface, and in part the water of springs, shallow or deep. In any case, a considerable amount of contact with the ground has taken place, and in consequence saline and organic matter is liable to be dissolved in a greater or less degree. The extent of the impregnation, as well as the kind of material dissolved, will depend on the rocks and strata of the river-basin.
River-waters, besides the qualities they derive from their primitive sources, are apt to contain mud, decayed leaves, the exuvhe of fish, and other matters in suspension, and are thus deficient in the clearness and transparency so essential to the satisfaction of the eye in a drinking-water. Moreover, the water. partakes of the extremes of sum mer and winter temperature. But the great objection to water from rivers is their general pollution from the manure used upon the land, sewage, and manufactures, so that there are now few rivers left from whose lower course a supply could be taken for domestic purposes. On the other hand, the supply from one of our large rivers is
boundless and unfailing; 'and it conveys the surface-drainage and spring effusions of a large tract of country, without incurring any trouble or expense as ,to the original sources. Rivers that issue from lakes are generally the purest, as the suspended matter has time to be precipitated.
Springs.—The qualities that recommend water to the eye and to the palate belong in a pre-eminent degree to spring-water (see SPRING): it is clear, sparkling, and of an agreeable and uniform temperature at all seasons of the year (about G0° Fahr.); it is well aerated, and is totally free from the offensive taint so common in all other waters, as well as devoid of the animalcules generated by organic impurity; and where a suffi cient number of springs can be collected to suffice for a town, it is the most desirable of all sources of supply. About a quarter of the water brought to Edinburgh is spring water collected on the slopes of the Pentlands.
Quality of Water.
Perfectly pure water is hardly to be found; rain-water, and even artificially distilled water, are only approximates. The chief impurities may be considered under the heads of mineral matter in supension, mineral matter in solution, and organic matter.
Mineral Matter in Suspension.—When running water comes upon a loose bottom, it car ries the finer particles of sand and earth along with it. If the water comes into a position of perfect stillness, the matters thus floated gradually sink to the bottom again. Parti cles of clay, owing either to their excessive fineness or to their adhesive attraction for 'later, subside very slowly, and impurities of this nature are not easily remedied. Besides earthy matter, compounds of iron and lead are also iu some circumstances pres ent in a solid state, and may be got rid of by filtering. To separate clay-powder from water, the practice has long been resorted to in India and China of putting in a piece of alum, which seems to produce a kind of coagulation.