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Rh  On the English scale, a water of 15° and over is hard, between 5" and 15° moderately hard, and of less than 5° soft.

That part of the hardness of a water which is actually owing to carbonate of lime (or magnesia) can easily be removed in two ways, (1) By boiling, the free carbonic acid goes off with the steam, and the carbonate of lime, being bereft of its solvent, comes down as a precipitate which can be removed by filtration, or by allowing it to settle, and decanting off the clear supernatant liquor. (2) A method of Clark's is to mix the water with just enough of milk of lime to convert the free carbonic acid into carbonate. Both this and the original carbonate of lime are precipitated, and can be removed as in the first case.

From any uncontaminated natural water pure water is easily prepared. The dissolved salts are removed by distillation; if care be taken that the steam to be condensed is dry, and if its condensation be effected within a tube made of a suitable metal (platinum or silver are best, but copper or block tin work well enough for ordinary purposes), the distillate can contain no impurities except atmospheric gases, which latter, if necessary, must be removed by boiling the distilled water in a narrow-necked flask until it begins to "bump," and then allowing it to cool in the absence of air. This latter operation ought, strictly speaking, to be performed in a silver or platinum flask, as glass is appreciably attacked by hot water. For most purposes distilled water, taken as it comes from the condenser, is sufficiently pure. The preparation of absolutely pure water is a matter of great difficulty. Stas, in his stoichiometric researches, mixed water with potassium manganate, and distilled after twenty-four hours; the product being re distilled and condensed in a platinum tube just before it was required.

Pure water, being so easily procured in any quantity, is used largely as a standard of reference in metro logy and in the quantitative definition of physical properties. Thus a "gallon" is defined as the volume at 62° F. of a quantity of water whose uncorrected mass, as determined by weighing in air of 30-in. pressure and 62° F of temperature, is equal to 10 lb avoirdupois. The kilogramme in like manner is defined as the mass of 1 cubic decimetre of water, measured at the temperature corresponding to its maximum density (4° C.). The two fixed points of the thermometer correspond—the lower (0° C., or 32° F.) to the temperature at which ice melts, the upper (100° C., or 212° F.) to that at which the maximum tension of steam, as it rises from boiling water, is equal to 760 mm. or 30-in. mercury pressure. 30 in. being a little more than 760 mm., 212° F. is, strictly speaking, a higher temperature than 100° C., but the difference is very trifling. Specific heats arc customarily measured by that of water, which is taken as =1. All other specific heats of liquids or solids (with one exception, formed by a certain strength of aqueous methyl alcohol) are less than 1. The temperate character of insular climates is greatly owing to this property of water Another physio graphically important peculiarity of water is that it expands on freezing (into ice), while most other liquids do the reverse. 11 volumes of ice fuse into only 10 volumes of water at 0° C.; and the ice-water produced, when brought up gradually to higher and higher temperatures, again exhibits the very exceptional property that it contracts between 0° and 4° C. (by about of its volume) and then expands again by more and more per degree of increase of temperature, so that the volume at 100° C. is 1,043 times that at 4° C.

In former times water was viewed as an "element," and the notion remained in force after this term (about the time of Boyle) had assumed its present meaning, although cases of decomposition of water were familiar to chemists. In Boyle's time it was already well known that iron, tin and zinc dissolve in aqueous hydrochloric or sulphuric acid with evolution of a stinking inflammable gas. Even Boyle, however, took this gas to be ordinary air contaminated with inflammable stinking oils. This view was held by all chemists until Cavendish, before 1784, showed that the gas referred to. if properly purified, is free of smell and constant in Its properties, which are widely different from those of air—the most important point of difference being that the gas when kindled in air burns with evolution of much heat and formation of water. Cavendish, however, did not satisfy himself with merely proving this fact qualitatively; he determined the quantitative relations, and found that it takes very nearly 1000 volumes of air to burn 423 volumes of "hydrogen" gas; but 1000 volumes of air, again, according to Cavendish, contain 210 volumes of oxygen; hence, very nearly, 2 volumes of hydrogen take up 1 volume of oxygen to become water. This important discovery was only confirmed by the subsequent experiments of Humboldt and Gay-Lussac, which were no more competent than Cavendish's to prove that the surplus of 3 units (423 volumes instead of 420) of hydrogen was an observational error. More recent work, e.g. of Morley, Leduc and Scott, has shown that the ratio is not exactly 2.1. The gravimetric composition was determined by Berzelius and Dulong, and later by Dumas by passing pure hydrogen over red-hot copper oxide. It has also been determined by several other variations and methods (see ).

The molecular weight of liquid water has attracted much attention, for it was perceived long ago that its high boiling point, refractive index and other properties were not consistent with the simple formula H2O. Cryoscopic measurements led to the probable formula (H-0)2, whilst the surface tension leads to (H2O)4. The question has been considered by H. E. Armstrong, who suggests that the simple molecule, H2O, which he calls hydrone, condenses in liquid water to form cyclic or chained compounds, containing tetravalent oxygen, resembling in structure the polymethylenes or paraffin's.

 WATER-BOATMAN, an aquatic hemipterous insect of the family Notonectidae, of which the best known species (Notonecta glauca) is a prominent feature in the pond-life of Great Britain. The technical name, Notonecta, meaning "back-swimmer," alludes to the habit of the insect of swimming upside down, the body being propelled through the water by powerful strokes of the hind legs, which are fringed with hair and, when at rest, are extended laterally like a pair of sculls in a boat. As is the case with other water-bugs, this insect is predaceous and feeds upon aquatic grubs or worms. The body is richly supplied with long hairs, which serve to entangle bubbles of air for purposes of respiration. The eggs are laid in the stems of water plants.

 WATERBUCK (Wasserbok), the name of a large South African antelope (Cobus ellipsiprymnus) belonging to the subfamily Cervicaprinae, characterized by the white elliptical ring on the Waterbuck. buttocks, and the general reddish grey colour of the long and coarse hair. They have heavily fringed necks and tufted tails, the bucks carry long sub-lyrate and heavily ringed horns, but the does are hornless. They seek refuge from pursuit in the water. The name is extended to include the sing-sing or defassa waterbuck (C. defassa), a widespread species, without the white ring on the buttocks, and represented by several local races, one of which is foxy red while a second is greyish. Both species equal in size the red deer. The smaller members of the genus Cobus (which is exclusively African) are generally called kobs. (See .)

 WATERBURY, a city and one of the county-seats of New Haven county, Connecticut, U.S.A., since 1900 coextensive with the township of Waterbury, on the Naugatuck river, in the west central part of the state, about 32 m. S.W. of Hartford. Pop. (1900) 51,139, of whom 15,368 were foreign-born (5866 being Irish, 2007 Italian, 1777 French Canadian, 1265 Russian, 1777 French, and 938 English); (1910 census) 73,141. Area 29 sq. m. Waterbury is served by the New York, New Haven & Hartford railway, and is connected by electric lines with New Haven, Bridgeport, Thomaston, Woodbury and Watertown. It has four public parks (the Green, Chase, Hamilton and Forest), with a total acreage of 80 acres, and a Soldiers' and Sailors' Monument, designed by George E. Bissell. The most important public buildings are the Federal building, the county court house, a state armoury, the Silas Bronson Public Library (1870, with an endowment of $200,000 and with 81,500 volumes in 1910), the Odd Fellows Temple, a Y.M.C.A. building and the Buckingham Music Hall (1907), and among the charitable institutions are the Southmayd Home (1898) for aged women, the Waterbury hospital (1890) and the St Mary's hospital (1908). In the city are the St Margaret's Diocesan School for Girls (Protestant Episcopal, 1875), the Waterbury Industrial School and the Academy of Notre Dame (1868). There is good water power here from the Naugatuck river and its tributaries Mad river and Great Brook. In 1905 Waterbury ranked third among the manufacturing cities of Connecticut (being surpassed only by