IMPURITIES OF WATER. =Properties of Water.=—When pure, water =is colourless=, or bluish when seen in large quantity. It should be quite =inodorous=. If, after keeping it for some time in a perfectly clean vessel, or if on heating it a smell is developed, the water is bad. Its =taste= should be pleasant and sparkling from the atmospheric gases dissolved in it. Bitterness generally indicates the presence of sulphate of magnesium (Epsom salts). Saltness is always a suspicious property, except in water obtained in the neighbourhood of salt mines or brine springs, or near the sea. It should be soft to the =touch=, and should dissolve soap easily. It should be bright and =clear=, and contain no suspended matters. Clear water is not necessarily pure, but turbid water is always to be rejected; the only exception being the brownish-tinged water from moors, which is not hurtful. In all other cases, printed matter should be legible through at least 18 inches of water in a clear glass cylinder. Thoroughly dissolved organic matter is less dangerous than suspended; the turbidity of water is therefore of great importance. But water may be bright and sparkling and apparently perfectly clear, and yet highly dangerous. The most important of the physical properties of water in regard to health are the absence of smell and turbidity, and these can be ascertained by even the most inexperienced. The chemical tests for the more important impurities are given (pages 85 to 87). The impurities of water may be classed under four heads—gaseous, mineral, vegetable, and animal. The gases ordinarily present in water cannot properly be regarded as impurities, inasmuch as they are always present, and greatly increase its palatableness. The dissolved nitrogen and oxygen bear to each other the proportion 1·42 to 1; where sewage contamination occurs, the oxygen will be diminished or disappear, owing to oxidation of the organic matter. The amount of carbonic acid gas in water varies greatly. It may be considerable in chalk waters, and in contaminated well-water.