Pure water is made up of two gases, - hydrogen and oxygen, - in the proportion of one part oxygen to two parts hydrogen by volume. It is represented by the symbol "H2O." Pure water is colorless, tasteless, and odorless. Pure water is never found in nature. Put a clean glass where the rain can fall unobstructed through the air into it during the latter part of a shower, and you will have water as nearly pure as it occurs in nature. Rain water absorbs gases of which the atmosphere is composed,- nitrogen and oxygen and carbon dioxide and a small amount of ammonia. It usually contains dust taken from the air, and may contain small amounts of soluble sub-stances, particles of which were floating in the air. That which flows over the roofs of buildings is likely to contain smoke, in addition to the other impurities. Rain water which flows over or through portions of the earth, as in the formation of rivers, lakes, and springs, takes up something in the form of soluble portions of soils, rocks, and decaying animal and vegetable matter found in its course. Many substances are soluble in water, and food nutrients needed by some plants are found in water, so that water which falls on a mountainous region has a very different composition when it reaches the sea, or an underground river or lake, from what it had when it first reached the earth. Rain water which falls in the latter part of a shower is purer than that which falls in the first part, and the water which falls in winter is purer than that which falls in summer.
Water is not a "nutrient," in the sense in which the word is generally used, but it is indispensable for nourishment, for it is a universal solvent, and a very convenient means of carrying the nutrients to the places where they are needed. It also carries waste products to places of exit, and conveys the surplus heat from the places where it is manufactured to the outside of the body, so that the temperature may be equalized, the blood purified, and the tissues built up or repaired. The frequent application of water to the outside of the body aids in the work of purification of the body by keeping the pores of the skin open, so that some of the waste products may be freely eliminated.
Perfect purity is not necessary in drinking water, but it should be free from visible particles, and should have no disagreeable taste or smell, either when fresh, or after it has stood for a time in a clean closed vessel. It should contain enough of the gases of the air to prevent its tasting flat, like distilled or long-boiled water. It should contain only a small amount of dissolved mineral matter, and this should not be of a poisonous nature, - as lead from pipes, etc. Water should be free from decaying animal or vegetable matter. Impurities of animal matter or the excreta of animals are usually more dangerous than those of vegetable origin. Water which has more than a trace of such matter is not safe. The purity of water cannot always be judged by its appearance, odor, or taste. Water which is clear and sparkling and tastes well may contain the germs of some dread disease; and water may contain the dead bodies of harmless confervae and Crustacea, minute sponges, etc., to such a degree as to cause it to smell bad for a time, without perceptibly injuring the one who drinks of it. If there is any question about the wholesomeness of water, boil it, and let it cool before using. So far as lies in your power, keep the surroundings clean. Pure water is "clean" in every sense of the word. Water in which sodium and magnesium salts are present in such small quantities as not to render it hard is desirable. When the conditions are such that the calcium or magnesium carbonate can be precipitated by continued boiling the water is called "temporary hard" water, because the hardness can be removed by boiling. The carbonate is not readily soluble in pure water, but is soluble in water containing carbon dioxide gas. Boiling drives the carbon dioxide gas off, and the calcium or magnesium carbonate is precipitated. The incrustation thus formed on the inside of the tea kettle can be removed by setting it out to freeze. When the calcium and magnesium occur as sulphates, they cannot be precipitated by boiling, and the water is known as "permanent hard" water.
A comparatively soft water is considered best for cooking some vegetables, as beans, peas, etc., as the hard water seems to have a less softening effect on them. In cooking some other vegetables, the kind of water used seems to make little difference. Moderately hard water is considered better than very soft water for making tea and coffee, because it dissolves less tannin, and the beverage is thus more wholesome. Soft water is far preferable for cleaning and for all laundry work, as it requires less soap and labor, and gives better results.