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ELEMENTS OF MATERIA MEDIC A.
Water enters into the composition of many minerals, either as water ofcrystallization , or combined as a hydrate.
( b .) In the organized kingdom , water is an essential constituent ofvegetables and animals.
Preparation. —Absolutely pure water may be procured by combiningits elements. For all practical purposes it is obtained sufficiently pure,by the distillation of common water. But water which has been re-peatedly distilled gives traces of acid and alkali when examined by theagency of galvanic electricity. Distilled water remains unchanged onthe addition of any of the following substances:—solutions of the causticalkalies, lime water, oxalic acid, the barytic salts, nitrate of silver, andsolution of soap. If any turbidness, milkiness, or precipitate, be occa-sioned by any of the above, we may infer the existence of some impurityin the water. But the most delicate test of the purity of water is gal-vanism, as before mentioned. The purest natural water is snow and rainwater; then follow river, spring, and well waters.
Properties. —Pure water has the following properties:—at ordinarytemperatures it is a transparent liquid, usually described as being bothodourless and colourless; but it is well known that the camel can scentwater at a considerable distance, so that to this animal it is odorous;and as regards its colour, we know that all large masses of water have abluish-green colour, though this is usually ascribed to the presence offoreign matters. When submitted to a compressing force equal to30,000 lbs. on the square inch, 14 volumes of this liquid are condensedinto 13 volumes; so that it is elastic. A cubic inch of water, at 00° F.,weighs 255'5 grains; so that this fluid is about 815 times heavier thanatmospheric air: but being the standard to which the gravities of solidsand liquids are referred, its specific weight is usually said to be 1\ At atemperature of 32°, it crystallizes, and in so doing expands. The funda-mental form of crystallized water (ice) is the rhombohedron. Water eva-porates at all temperatures, but at 212° boils, and is converted into steam,whose bulk is about 1700 times that of water, and whose sp. gr. is O'024(that of hydrogen being 1). Water unites with both acids and bases,but without destroying their acid or basic properties. Thus the crystal-lized vegetable acids, tartaric, citric, and oxalic, are atomic combinationsof water with acids. Potassa fusa and slacked lime may be instanced ascompounds of water and basic substances: these are called hydrates.It is a chemical constituent of some crystallized salts; for example,alum, sulphate of soda, and sulphate of magnesia. Here it exists aswater of crystallizatian. It rapidly absorbs some gases—as fluoride ofboron, ammonia, &c. It is neither combustible nor a supporter of com-bustion.
Characteristics. —In the liquid state it is recognized by being vola-tile, tasteless, odourless, neither acid nor alkaline, and not combustiblenor a supporter of combustion: it is miscible with alcohol, but not withthe fixed oils ; if potassium be thrown on it in the open air, the metaltakes fire. Lastly, water may be decomposed into oxygen and hydrogenby the galvanic agency. The most delicate test of aqueous vapour inany gas, is fluoride of boron (commonly called fluoboric acid gas), whichproduces white fumes with it.
Composition —The composition of water is determined both bvanalysis and synthesis. If this liquid be submitted to the in flu-