WATER. 334 WATER. due in part to the varying amounts of carbonic acid contained by them, although the nature of the bottom and the color of the sky largely de- termine the color of -n-aters, be.vond doubt. Liquid water is a poor conductor of heat, and when per- fectly pure is one of the poorest known conduc- tors of electricity. The purification of water for scientific purposes is best effected by distilla- tion. An efiicient method consists in dissolving a small amount of potassium permanganate in a (piantity of ordinary water, allowing it to stand for a day, then adding a further amount of potassium permanganate and some caustic potash and distilling, preferably in an apparatus pro- vided with a platinum condenser {the metal of the apparatus must be free from lead) ; the first portions of the distillate should be rejected, and the distillation should be interrupted after about four-fifths has passed over; the intermediate portion may be further purified by dissolving in it a small amount of acid potassium sulphate and redistilling. Natural waters may be classi- fied according to their origin. Rain v:ater, which forms by the condensation of the aqueous vapor more or less abundant in the air, is the purest of all natural water, although it takes up for- eign substances in its passage from the clouds to the earth, and always contains ammoniacal salts, sodium chloride, and other inorganic salts, as well as traces of organic matter of various kinds. Rain water collected in places where much coal is burned invariably contains traces of sulphuric acid derived from the oxidation of the sulphur from the pyrite present in most coals. Surface water, which includes the water from rivers, is of course less apt to be ])ure tlian rain water, its character being determined largely by the nature of the soil over which it passes, and by the vegetation on the surface. The min- eral ingredients are usually carbonates, chlorides, and sulphates of the alkalies and alkaline earths, with small quantities of iron and manganese, while the organic matter is of vegetable origin. Well water is that derived from wells or bore- holes made in the surface of the earth, and in- cludes the water from shallow wells and that from deep wells. The shallow wells serve chiefly as the source of domestic supply, especially in country districts, and their water is liable to contamination from sewage and other refuse animal matter, owing to the fact that they are invariably sunk in the immediate vicinity of human dwellings. It is to such sources that epidemics of typhoid fever and similar diseases are often traced. Nitrates, nitrites, and am- monia arc the constituents commonly found in such waters, and their presence indicates con- tamination of a dangerous character. Wells more than 100 feet in depth, and cspceially artesian wells, arc used as sources of water, chiefly in cities, for manufacturing plants, which contains only mineral ingredients. Mineral waters pos- sess more or less marked medicinal properties, in conseqiience of the considerable amounts in them of substances capable of exercising various physiological eflTects. Such waters are classed according to their temperature into cold, tepid, warm, or hot, and according to their ingredients into alkaline, alkaline-saline, saline, and acid waters. The alkaline waters are those that con- tain the alkaline carbonates, and, owing to the presence of free carbon dioxide, are sometimes designated as 'carbonated waters.' The alkaline- saline waters include those in which there is a mixture of carbonates with sulphates or chlorides, and are subdivided according to the predominance of the acid constituent into sulphated, muriated, and borated. The saline waters are those which contain a predominance of sulphates or chlorides, and are similarly subdivided into sulphated, muriated, and borated; while the acid waters are those that contain free sulphuric, hydrochloric, or silicic acid. Sometimes these four classes are further subdivided according to their pre- dominant basic solid constituents; thus when the sodium or lithium salts predominate, the water is known as sodic or litliic, etc. Sea water is the water which forms the ocean ; and as it is constantly receiving the waters more or less impure from rivers, and at the same time losing pure water in the form of vapor by means of evaporation, the impurities remain behind, and have given to this water its well-known saline character. An analysis of sea water shows traces of nearly every known element, and its salinity varies in different parts of the world. When it freezes the resulting ice is nearly, though not quite, free from salt. W'aters are distinguished as hard and soft according as they contain large or small quantities of calcium or magnesium salts in solution, and this fact may be deter- mined by an examination with a soap solution, the addition of which causes the formation of lather when the quantity added is sufficient to have rendered the water soft. Ice or Solid Water. The passage of water from the liquid to the solid state is accompanied by an expansion equal to about one-eleventh of its vol- ume ; hence water freezing in pipes may easily cause them to burst. Tlie normal freezing-point of water varies but sliglitly with the external pressure : considerable pressures, however, cause a marked depression of the freezing-point (q.v.). Besides, if carefulh' kept out of contact with the air, water may be readily under-cooled to temperatures far below its freezing-point ; it is then, however, in an exceedingly unstable state, and mere contact with a sharp-edged body, or especially exposure to the air, causes the entire mass to freeze very rapidly, the temperature simultaneously rising to the normal freezing- point. The heat developed during the freezing of one gram of water is 79.06 calories. Natural ice is transparent and highly coherent. Ice of similar properties is obtained artificially when the cooling medium employed is only about one degree below the freezing-point : the ice formed at lower temperatures occludes minute globules of air, and these render it whitish and more or less ojiaque. W.TER Vapor, or Gaseous W'ater. This is a normal eonstitient of the atmosphere (q.v.). The passage of one gram of liquid water into steam at 100° C. (212° F.) involves the absorp- tion of as much as 536 calories of heat. At every temperature water-vapor can only exist below a certain maxinnnn of pressure, viz. the vapor tension of water at that temperature. ITnder that pressure the vapor is said to l)e 'saturated.' Stronger pressure causes lii|Ucfaction, unless the femperature is above the crifical point, which, according to Nadcjdin, is about 2.'>H° C. (about 4'.H'iIi° v.). (See rniTlCAi, Porv'T.) Wafer vapor is colorless and perfectly transparent, unless it
