VARIOUS STATES OF WATER IN COMBINATION. 37 and assumes a variety of crystalline forms derived from the rhombohedron and six-sided prism. Water evaporates at all temperatures, and under the ordinary pressure of the atmosphere it boils at about 2120. Its anomalous expansion by heat (143), and the important purposes thereby attained (151), as well as the great dilatation which it undergoes on freezing (76), have been already pointed out. Arago and Fresnel have shown, that notwithstanding the gradual dilatation of water at temperatures below 39°, its refractive power on light continues to increase regularly, as though it contracted. Its density at 60° is taken as 1'000, and it forms the standard with which, in this country, the specific gravities of all solids and liquids are compared. A cubic inch of water at 60° F. weighs in air 252.456 grains, and a cubic foot very nearly 1000 (more exactly 997) ounces avoirdupois. To the chemist water is invaluable as a solvent. It is the perfection of a neutral substance; and it enters into combination most extensively both with acids and with bases. Experience has shown that when an anhydride, or so-called anhydrous acid, has once been allowed to combine with water, the entire separation of the water from the compound is often impracticable, unless some powerful base be presented to the acid; in such a case the base appears to displace the water, and its expulsion by heat is then easily effected. Suppose, for example, that sulphuric acid has been freely diluted with water; upon the application of heat the water at first passes off readily, leaving the less volatile acid behind. By degrees, however, it becomes necessary to increase the temperature in order to expel the water, and at last the acid begins to evaporate also, and finally no further separation can be effected, because when the temperature rises to about 640° F. the entire liquid distils over. It is found on analysing the acid when it has reached this point, that the composition of the liquid may be represented by the formula H2SO,, or HO,SO,. But if to this concentrated acid a base, such as oxide of lead, be added, the water is easily expelled, and anhydrous sulphate of lead is obtained : Upon the older and still current view of the constitution of salts, which regards these bodies as formed by the union of an anhydride with a base, the water would in the foregoing instances supply the place of a base, and it hence has been termed basic water, e.g. :HO,SO, oil of vitriol. PbO,SO,, sulphate of lead. 3 38 WATER IN COMBINATION. Still adopting the older view, it has been supposed in a similar manner that water combines with the powerful bases, such as potash or soda, and then cannot be expelled from them until some acid has been added. Potash in the form in which it is obtained by evaporating down its aqueous solution and heating the residue to dull redness, contains the elements of one equivalent of the alkali and one of water (KO,HO): this equivalent of water cannot be expelled except by the addition of an acid, such as sulphuric acid; then by the application of heat, anhydrous sulphate of potash is obtained. In such a case the water in combination with the base appears to perform the part of an acid. The foregoing explanation is inadmissible if water be represented as consisting of H,O; but in this case the presence of hydrogen in hydrate of potash may be equally well accounted for, if it be supposed that caustic potash is a compound formed upon the same plan or type as water, but that it contains an atom of potassium in place of one of the atoms of hydrogen present in the molecule of water. Further, anhydrous potash, which may be formed from the hydrate of the alkali by heating it with potassium, whilst hydrogen is liberated, is viewed as containing two atoms of potassium in place of the two atoms of hydrogen in the molecule of water. The relations of these three different compounds may be thus represented : The reaction of sulphuric acid upon hydrate of potash is then a true case of double decomposition, as may be thus represented : Hydrate of Sulphuric Water. Sulphate of 2 ᏦᎻᎾ + Ꮋ ᎦᎾ, = 2 ᎻᎻᎾ + Ꮶ ᎦᎾ, the two atoms of potassium of the base, and the two of hydrogen in the acid, changing places with each other, whilst sulphate of potassium and water are each formed simultaneously. The compounds of water are frequently termed hydrates. When a body is described as being entirely free from water, in combination, it is commonly said to be anhydrous (from a not, ὕδωρ water). Many salts in crystallizing unite with a definite quantity of water, which is essential to the form of the salt, but which may, by the application of a gentle heat, be expelled without altering the chemical properties of the saline body. In this case the WATER IN COMBINATION.-NATURAL WATERS. 39 water is spoken of as water of crystallization. Many salts part with such water by mere exposure to air. Carbonate of sodium, for example, crumbles down or effloresces to a white powder; and the same thing occurs in the case of sulphate of sodium.* The form of the salt depends upon the quantity of this water of crystallization. For instance, borax is always found to crystallize with 10 atoms of water (Na,B,. 10 H2O), in oblique rectangular prisms, if the solution of the salt be not sufficiently concentrated to begin to crystallize till the temperature falls to 133° F.; but from a more concentrated solution borax is deposited in regular octohedra with only 5 atoms of water. So, again, the sulphate of sodium crystallizes, under ordinary circumstances, in oblique four-sided prisms with 10 atoms of water (Na2SO4· 10 H2O); but if a solution saturated at 91°, be very slowly raised to 2120, the sulphate of sodium is deposited in rhombic octohedra which contain no water. (344) Various kinds of Natural Waters.-Owing to its extensive solvent powers, water is never met with naturally in a state of purity. Rain water, collected after a long continuance of wet weather, approaches nearest to it, but even that always contains atmospheric air, and the gases floating in the air, to the extent of about 2 cubic inches of air in 100 of water.† Spring Water, although it may be perfectly transparent, always contains more or less of saline matter dissolved in it; the nature of these salts will of course vary with the character of the soil through which the water percolates. The most usual saline impurities are carbonate of calcium, common salt, sulphate of calcium, and sulphate and carbonate of magnesium. The waters of the New Red Sandstone are impregnated to a greater or less extent with sulphate of calcium. Most spring waters are charged with a notable proportion of carbonic acid, which dissolves a considerable amount of carbonate of calcium; the calcareous springs in the chalk districts around London contain from 18 to 20 grains of chalk per gallon, 6 or 8 grains of which have become separated by exposure of the water to the atmosphere, so that a running stream will seldom contain more than 12 or 14 grains of chalk per gallon in solution. Waters which have filtered through a bed of chalk also often contain carbonate of sodium in consi * Other salts, on the contrary, absorb moisture from the atmosphere, and become damp or even liquefy in the water so absorbed; they are then said to deliquesce. Carbonate of potassium and chloride of calcium offer instances of this kind. †The quantity of air which is contained in spring or other water can be 40 SEPARATION OF AIR FROM SOLUTION IN WATER. derable quantity, as is the case with the deep-well waters of London. Mineral Waters are waters impregnated with a large proportion of any one of the above-named salts, or with some substance readily ascertained in the following manner. A globular flask, a, fig. 276, FIG. 276. capable of containing 14 wooden vice, such as is seen at f, is made use of to compress the vulcanized tube and to cut off communication between the flask and the bulb, c. The water in c is now made to boil briskly for ten minutes or a quarter of an hour, until all the air is expelled from the tube, the mouth of which is kept just below the surface of the mercury. When, after the boiling has been continued for a few minutes, no more air escapes from the tube, the jar, d, is filled with mercury and placed over the end of the long tube. The vice is removed, and heat applied to the flask; the water speedily begins to give off gas; and the quantity increases till the water boils. The ebullition must be continued steadily for a full hour, and the operation terminated by a few minutes' brisk boiling, by which the delivery tube will be filled with steam, and all the air will be driven over into the jar. One object of the globe, c, is to prevent the water from boiling over into the jar, d: a little steam always condenses in the jar above the mercury, but this is a matter of small consequence. When the operation has terminated, the gas is allowed to cool, and is transferred to a tall jar of water, or of mercury, where its bulk can be measured. It will be found that all water, including even that which has been recently distilled, contains air. For example, three samples of water twice distilled in glass vessels, were submitted to experiment: 100 cub. in. of the first specimen contained 185 cub. in. of air; in the same bulk of the second 2:15, and the third specimen 2.38 cub. in. of air were present; the oxygen and nitrogen being in each case almost exactly in the proportion of 1 measure of oxygen to 2 measures of nitrogen. MINERAL WATERS-RIVER WATER. 41 not so commonly met with: such waters are usually reputed to possess medicinal qualities, which vary with the nature of the salt in solution. Many of these springs are of a temperature considerably higher than that of the surface of the earth where they make their appearance. At Carlsbad and Aix-la-Chapelle this temperature varies from 160° to 190°. Such hot springs either occur in the vicinity of volcanoes, in which case they generally abound in carbonic acid, as well as in common salt and other salts of sodium: or they spring from great depths in the rocks of the earliest geological periods, and contain chlorides of calcium and magnesium, and almost always traces of sulphuretted hydrogen. (Berzelius.) Many mineral waters contain salts of iron in solution, which impart to them an inky taste; they are then frequently termed chalybeate waters; some of the Cheltenham springs are of this kind. In other instances carbonic acid is very abundant, giving the brisk effervescent character noticed in Seltzer water. Less frequently, as in the Harrogate water, sulphuretted hydrogen is the predominating ingredient, giving the nauseous taste and smell to such sulphureous waters. In other instances the springs are merely saline, and contain purgative salts, like the springs at Epsom, which abound in sulphate of magnesium, and at Cheltenham, where common salt and sulphate of sodium are the predominant constituents. Many of these saline springs also contain small quantities of iodine and bromine, which add greatly to their therapeutic activity. River Water is less fitted for drinking than ordinary spring water, although it often contains a smaller amount of salts; for it usually holds in solution a much larger proportion of organic matter of vegetable origin, derived from the extensive surface of country which has been drained by the stream. If the sewerage of large towns situated on the banks be allowed to pass into the stream, it is of course still less fit for domestic use. Running water is, however, endowed with a self-purifying power of the highest importance; the continual exposure of fresh surfaces to the action of the atmosphere promotes the oxidation of the organic matter, and if the stream be unpolluted by the influx of the sewerage of a large town, this process is generally fully adequate to preserve it in a wholesome state. River water almost always requires filtration through sand before it is fit for domestic use; and if water-works designed to supply such water be properly constructed, provision is made for this filtration. Suspended matters, such as weeds, fish-spawn, leaves, and finely divided silt or mud, |