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soap, sulphate, acid, magnesium, hard, calcium, substances, hardness, sodium and boiling

WATER (Formula, OH,).—A clear colorless liquid devoid of taste and smell and possessing a neutral reaction. Pure water is tasteless and colorless, except in thick layers. It boils at ioo C. (212 Fahr.), and becomes solid at o C. (,32 Fahr.) It is undoubtedly one of the most important substances in nature; it is also the most widely distributed, occurring in three physical states, z.e., liquid, gaseous and solid. In its liquid state it forms the oceans, rivers, lakes, which occupy nearly three-fourths of the whole surface of the earth. In its solid state it forms ice and snow, existing permanently at the polar regions, and, as a vapor, is an important constituent of the aerial envelope of the earth.

Pure water is not found in nature, owing to the fact that it dissolves a portion of whatever it comes in contact with. Rain is the purest form of natural water, but contains certain gases, which: it collects from the atmos phere during its descent to the earth.

Water taken from wells, springs, rivers, lakes, etc., differs very much, according to the nature of the earth or rocks over which it passes, or upon which it lies. Among the various sub stances found dissolved in water taken from different places may be mentioned the following :f Sodium sulphate, sodium chloride, magnesium sulphate, magnesium chloride, magnesium car bonate, calcium sulphate, calcium carbonate, sulphuretted hydrogen, potassium sulphate, potas sium chloride ; phosphates, bromides and iodides of calcium, aluminum, sulphate, ferrous car bonate, carbonic acid, silica, and a large variety of vegetable substances.

Water from natural sources is commonly classified as hard and soft, terms which are applied to it according to the manner in which it acts upon soap. If, for instance, we rub a piece of soap in "soft" water, such as ordinary rain water, it is not a difficult matter to speedily produce a lather, and its cleansing powers arc easily made use of; but if on the other hand, the water is what is termed "hard," spring water for instance, it will be found that the soap requires a much longer time and considerably more friction to produce the required lather. Curdy flakes will also make their appearance. The reason of this is that soap is formed by the combination of a fatty acid, with an alkali manufactured by boiling oils and fatty substances, with potash for soft soap, and with soda for hard soaps. The action of the soda is to rob from the oil or fat two acids, stearic and oleic acid, and with which it unites to form hard soap, which would be chemically termed a mixture of stearate and oleate of sodium. If we take some soapy water and add a little magne sium sulphate (Epsom salts) solution to it, curdy flakes will be produced in just the same man ner as when soap is rubbed in hard water, the soap losing its properties of frothing. The magne sium sulphate has decomposed the soap, forming sodium sulphate, which remains dissolved in the water, and insoluble curdy flakes consisting of stearate and oleate of magnesium. A similar effect is produced by the hard water, the hardness being caused by the presence of different calcium and magnesium salts, all of which have a decomposing action upon the materials contained in the soap. If the hardness be caused by the presence of calcium bi carbonate, it can be got rid of by boiling, as can also most of that due to magnesium bi carbonate. The effect of boiling is to drive away the carbonic anhydride that causes these salts to become soluble, and they will be precipitated.

When spring, river, or well water is boiled in a kettle, a thick fur or incrustation is soon formed in the interior of the vessel. This is usually of a brown color, and the harder the water the more rapid will be the deposit. The explanation is that when natural water is boiled the carbonic acid gas contained in it is expelled, and, as the calcium, magnesium, and iron carbonates are not soluble in water which does not contain carbonic acid, they are precipitated, forming a succession of those layers on the bottom and sides of the vessel in which the water is boiled. This deposit must be removed, and is very annoying in the case of steam boilers, as it is a bad conductor of heat, and in consequence a great waste of fuel is occasioned. A small marble placed in the kettle will lessen the amount of deposit upon the sides. It will be understood that when water is boiled for some time it becomes much softer, owing to the precipitation of the salts. When the hardness can he removed by boiling, it is termed " temporary hardness," but if due to such substances as sulphate of lime, which is not thrown out of solution by boiling, it is termed " permanent hardness." The organic matter contained in water may be due to several causes. It may be dis solved from the earth, or it may be due to the decomposition of animal or vegetable substances, or from sewage or drainage which has gained access to it.

There are a number of fine analytical tests for the purity of water, used by chemists, which are, however, far too elaborate for the ordinary photographer. The following simple tests may, however, be found useful.

I.—Pour about half a pint of water into a wide-mouth bottle or decanter, close it with the stopper or with the palm of the hand, and shake violently up and down. If an offensive odor is then detected, the water is probably contaminated with sewage gas.

2.—Add to a little water a drop or two of dilute sulphuric acid and enough potassium permanganate (Condy's fluid) to tinge it of a faint rose color, and cover the vessel with a glass plate or a saucer. If the pink tinge be still visible after a quarter of an hour or so, the water is probably good.

3.—Pour a little solution of silver nitrate into a carefully cleaned glass, and see that it remains transparent ; then pour in some of the water. Should a strong milkiness appear, which is not cleared up on adding a little dilute nitric acid, the water probably contains much sodium chloride.

The purest form of water is obtained by distillation, that is, by first of all converting the liquid into a vapor, and then recondensing it into the liquid form again. (See Water, Distilled).

• WATER, DISTILLED.—Pure water obtained by distillation. The water is first filtered, and then distilled in a still (q.v.) The water which comes out first should be rejected, as it con tains nearly all the volatile impurities ; and if one-tenth of the water is allowed to remain in the retort the solid impurities are also left.

Distilled water should give no precipitate with silver nitrate, showing the absence of chlorides ; nor with ammonium oxalate, showing the absence of lime ; nor with barium chloride, showing the absence of sulpuric acid. A drop of potassium permanganate should give a permanent pink tint to the water, proving the absence of organic matter