WATER 399 less considerable strata of earth, are, as a class, clear of suspended, but rich in dissolved, mineral matter. Of ordinarily occurring minerals only a few are perceptibly soluble in water, and of these carbonate of lime, sulphate of lime, and common salt are most widely diffused. Common salt, however, in its natural occurrence, is very much localized ; and so it comes that spring and well waters are contaminated chiefly with carbonate and sulphate of lime. Of these two salts, however, the former is held in solution only by the carbonic acid of the water, as bicarbonate of lime. But a carbonate-of-lime water, if exposed to the atmosphere, even at ordinary temperatures, loses its carbonic acid, and the carbonate of lime crystallizes out. The fantastically shaped " stalactites " which adorn the roofs and sides of certain caverns are produced in this manner. In the relatively rare cases where a spring water in the course of its migrations meets with a deposit of common salt or other soluble salts, it dissolves more or less of these and becomes a salt-water. Most salt-waters are substan tially solutions of common salt (chloride of sodium), associated with only little of salts of potash and magnesia. But there are exceptions; in the so-called " bitter waters" the dissolved matter consists chiefly of sulphate of magnesia and other magnesia salts. Immense quantities of carbonic acid gas are being con stantly produced in the interior of the earth, probably by the action, at high temperatures, of steam on the carbonates of lime and magnesia. Some of it collects and is stored up temporarily in cavities, but the bulk streams out into the atmosphere, invisibly as a rule, through what one might call the capillaries of the earth s body; but here and there it unites into veins and arteries and comes forth, it may be, as a mighty carbonic acid spring. Carbonic acid being one and a half times as heavy as atmospheric air, it may collect into pools or even lakes ; the " Dog s Grotto," near Naples, and the " Valley of Death," in Asia Minor, are illustrative examples ; but, carbonic acid being a gas, and consequently diffusible, such lakes can maintain their level only if there is a constant and abundant supply of the gas from below. A far more frequent occurrence is that a mass of water and a mass of carbonic acid meet within the earth. As a rule, the pressure on the gas is more than one atmosphere, and the supply of the gas is abundant. The water then takes up considerably more carbonic acid than it would under ordinary atmospheric pressure, and if an outlet be provided, perhaps artificially by a boring, a frothy mass of carbonic-acid water comes forth as a fountain, sometimes of great volume. Such carbonic-acid waters are met with in many parts of the world, chiefly, however, in Germany. The well-known Apollinaris water is an example. In this connexion we may refer also to the soffioni of Tuscany jets of steam charged with ammonia or vapour of boric acid, which condense in the air and collect into " lagoons " of (substantially) a very dilute solution of boric acid. Boric acid waters, however, appear to be entirely confined to a limited district in Tuscany to the south of Yolterra. In addition to its natural components, water is liable to be con taminated through accidental influxes of foreign matter. Thus, for instance, all the Scottish Highland lochs are brown through the presence in them of dissolved peaty matter. Hirers flowing through, or wells sunk in, populous districts may be contaminated with excrementitious matter, discharges from industrial establish ments, &c. Our instinct rebels against the drinking of a contam inated water, and it guides us correctly. Not that those organic compounds are in themselves hurtful. An otherwise pure water, contaminated with, say, one ten thousandth of its volume of urine, might be drunk with perfect confidence. Yet the presence of especially nitrogenous organic matter is a serious source of danger, inasmuch as such matter forms tho natural food or soil for the development of micro-organisms, including those kinds of bacteria which are now supposed to propagate infectious diseases. Happily nature has provided a remedy. The nitrogenous organic matter dissolved in (say) a river speedily sutlers disintegration by the action of certain kinds of bacteria, with formation of ammonia and other (harmless) products of simple constitution ; and the ammonia, again, is no sooner formed than, by the conjoint action of other bacteria and atmospheric oxygen, it passes first into (salts of) nitrous and then nitric acid. A water which contains combined nitrogen in the form of nitrates only is, as a rule, safe organically ; if nitrites are present it becomes liable to suspicion ; the presence of ammonia is a worse symptom ; and if actual nitrogenous organic matter is found in more than microscopic traces the water is possibly (not necessarily) a dangerous water to drink. Wanklyn s method of water analysis is based upon these ideas. Starting with half a litre of the water, he distils it with a small quantity of carbonate of soda, and in the distillate determines the ammonia colorimctrically with Nessler s reagent (see MERCURY, vol. xvi. p. 34). When all the saline ammonia (the/rce ammonia) is thus eliminated, alkaline permanganate of potash is added to the residue and the distillation resumed. Part of the nitrogen of the organic matter is eliminated as ammonia ; it is determined in the distillate, by Nessler s reagent, and reported as albumcnoid am monia. The results are customarily referred to one million parts of water analysed. To give an idea of the order of quantities here dealt with, let us say that a water yielding O l part of free and O l part of albumenoid ammonia would be condemned by any chemist, j as possibly dangerous. The peaty waters of the Scottish Highlands, i however, contain much of both ammonias, and yet are drunk by the 1 natives with perfect impunity. All waters, unless very impure, become safe by boiling, which process kills any bacteria or germs that may be present. Of the ordinary saline components of waters, soluble magnesia and lime salts are the only ones which are objectionable sanitarily if present in relatively large proportion. Carbonate of lime is harmless ; but, on the other hand, the widely diffused notion that the presence of this component adds to the value of a water as a drinking water is a mistake. The farinaceous part of food alone is sufficient to supply all the lime the body needs ; besides, it is
- questionable whether lirne introduced in any other form than that
i of phosphate is available for the formation of, for instance, bone tissue. The fitness of a water for washing is determined by its degree of softness. A water which contains lime or magnesia salts decom poses soap with formation of insoluble lime or magnesia salts of the fatty acids of the soap used. So much of the soap is simply wasted ; only the surplus can effect any cleansing action. An excellent and easy method for the determination of the hardness of a water has been devised by Clark. A measured volume of the water is | placed in a stoppered bottle, and a standard solution of soap is then dropped in from a graduated vessel, until the mixture, by addition of the last drop of soap, has acquired the property of throwing up a peculiar kind of creamy froth when violently shaken, which shows that all the soap-destroying components have been pre cipitated. The volume of soap required measures the hardness of the water. The soap-solution is referred to a standard by means of a water of a known degree of hardness prepared from a known weight of carbonate of lime by converting it into neutral chloride of cal cium, dissolving this in water and diluting to a certain volume. A water is said to have " 1, 2, 3, . . . n degrees of hardness," if it is equivalent in soap-destroying power to a solution of carbonate of lime containing 1, 2, 3, ... n grains of this salt per imperial gallon. That part of the hardness of a water which is actually owing to carbonate of lime (or magnesia) can easily be removed in two ways. (1) By boiling, the free carbonic acid goes off with the steam, and the carbonate of lime, being bereft of its solvent, comes down as a precipitate which can be removed by filtration, or by allowing it to settle, and decanting off the clear supernatant liquor. (2) A method of Clark s is to mix the water with- just enough of milk of lime to convert the free carbonic acid into carbonate. Both this and the original carbonate of lime are precipitated, and can be removed as in the first case. See p. 409 infra. From any uncontaminated natural water pure water is easily pre pared. The dissolved salts are removed by distillation ; if care be taken that the steam to-be condensed is dry, and if its condensation be effected within a tube made of a suitable metal (platinum or silver are best, but copper or block tin work well enough for ordinary purposes), the distillate can contain no impurities except atmospheric gases, which latter, if necessary, must be removed by boiling the distilled water in a narrow-necked flask until it begins to "bump," and then allowing it to cool in the absence of air. This latter operation ought, strictly speaking, to be performed in a silver or platinum flask, as glass is appreciably attacked by hot water. For most purposes distilled water, taken as it comes from the con denser, is sufficiently pure. Pure water, being so easily procured in any quantity, is used largely as a standard of reference in metrology and in the quantita tive definition of physical properties. Thus a "gallon" is defined
as the volume at 02 F. of a quantity of water whose uncorrected