Aqual Communes. Aqua nivalis, plu-vialis, fluvialis, fontana. Common waters: snow, rain, river, spring waters.

It is needless to observe, how much the purity of waters is conducive to health; and how greatly, though by insensible degrees, the human body must necessarily be affected, by minute quantities of insalubrious matters in this universal diluent, and vehicle of all our aliment.

Among the common tests of the purity of water, the least fallacious are, its being perfectly colourless, transparent, and void of smell and taste; its dissolving soap into a smooth lather; boiling pulse tender; not changing the colour of syrup of violets or the juices of other blue flowers; and its mingling with alkaline and with acid liquors, with solution of sulphur in alkalies, and solution of silver in the nitrous acid, without precipitation or change of trans-parency. These trials serve to distinguish, in most cases, whether waters contain any con-siderable quantity of foreign matters, but what the particular matters are, they never can dis-cover, different substances exhibiting, in the several experiments, similar phenomena: thus blue juices are changed red by alum as well as by acids, and green by the calcareous marine salt as well as by alkalies.

To determine, with any degree of precision, the contents of waters, a quantity of the water is to be evaporated, in clean glass vessels, with a heat scarcely exceeding that which the hand can support; that the solid contents may be procured by themselves, with as little danger as possible of the extrication or transposition of any of their principles. The dry matter being digested in a little pure distilled water, a saline substance is commonly extracted by the water, and an earthy one is left, no longer dissoluble in aqueous menstrua.

The earthy matter is commonly not one simple earth, but a combination of two or more: 1. aluminous earth, distinguished by its dissolving in the vitriolic acid into an austere liquor; 2. magnefia, dissolving in the same acid into a bitter liquor; 3. calcareous earth, not dissolving at all in the vitriolic acid, but readily in the nitrous and marine, from both which it is precipitated by the vitriolic; 4. selenites, not dissoluble in any acid, till strongly calcined in contact with burning fuel, by which process it is reduced to calcareous earth; 5. some of the absolutely indissoluble earths, whose particular species, in the small quantities wherein they are obtained in these kinds of experiments, it is difficult and of little importance to determine. The two first are rarely met with in the residua of waters; the others are frequent, perhaps universal.

The saline substances are; the mineral fixt alkali, natron-, the vitriolic acid, combined with this alkali into sal mirabile, or with mag-nesia into sal catbarticus, or with the aluminous earth into alum; the nitrous acid, combined with the alkali into nitre (a), or with some of the soluble earths into nitrous salts; the marine acid, combined with the alkali into common salt, or with soluble earths into muriatic salts, or a volatile alkali combined with the acids into ammoniacal salts (see the respective salts) * (b).

(a) Leigh, natural hist. of Lancashire, etc. p. 39.

* (b) Another medium by which mineral substances may be rendered soluble in water is fixable air, or gas. This matter is found to be contained in large quantity in many mineral waters, particularly such as are distinguished by a brisk sharp taste, and the property of sparkling when poured out. From some experiments of the Hon. Mr. Cavendifh (Philof. Tranf. Vol. Lv11.) it was found, that calcareous earth could be dissolved in water impregnated with fixable air; and it appeared probable, that this combination actually takes place in the composition of Rath-bone-place water. Mr. Lane, in Vol. LIX. of the Philos. Trans. has proved by experiment, that the same kind of gas will render iron soluble in pure water, without any other addition; whence it may be concluded with sufficient probability, that a part at least of the iron in the chalybeate waters of Spa and Pyrmont, which are known to abound with fixable air, is held in solution by means of this principle. This supposition well explains several of the phenomena of these waters; as particularly their speedy decomposition in the open air, when this very volatile principle is dissipated. The above conclusions are confirmed by Dr. Brownrigg's experimental inquiry into the nature of the elastic spirit contained in the Pouhon water and other acidulae. Philos. Trans. Vol. LV. and LXIV. In order to detect the presence of this matter in waters, some of the water must be boiled in a close vessel, from which a tube is carried, having a bladder tied to its extremity. The air expelled by the heat will enter the bladder, and may then be examined by the proper tests of fixable air. Vid. Aer fixus.

The common, muriatic and nitrous salts are frequent; nitre, alum, sal mirabile, and amnioniacal salts very rare.

Most of these salts may, by careful crystalli-zation, be separated in their proper form: they may likewife be distinguished, however blended together, by additions. 1. The fixt alkali, unsaturated, is known by its raising an effer-vescence with spirit of salt. 2. The apecies of acid is distinguished, by adding to the exsicca-ted mass a little oil of vitriol: if the acid is the marine, it will be expelled in white, and if the nitrous, in red vapours; but if it is the vitriolic, no change will ensue. The marine acid may likewise be known, by the compound enabling pure aqua fortis to dissolve gold leaf, or a mark made with gold on a touchstone; the nitrous, by its deflagrating, when ignited, on the contact of any inflammable matter; the vitriolic, by its precipitating any solution of calcareous earth, as of chalk in aqua fortis, and the precipitate being a felenites, or not disso-luble in fresh aqua fortis. 3. The basis, or substance combined with the acid in the saline compound, is found, by adding to a solution of the matter, a little solution of salt of tartar, or any other fixt alkaline salt: if the basis is a fixt alkali, no change will ensue; if an earth, it will precipitate; if a volatile alkali, a pungent smell will discover it.

The purest of the common waters is that of snow, carefully collected on the tops of mountains, or on an open plain. A gallon, slowly evaporated or distilled, leaves only two or three grains of solid matter. Distilled water itself leaves nearly as much, upon a second, and upon repeated distillations; but with this difference, that the residuum of snow water, like that of all the other natural ones, is brownish and saline; whereas, that of the distilled is a fine white earth, void of saline matter, partly calcareous, and partly indissoluble. Snow water, kept in a warm place, in clean glass vessels, not closely stopt but covered from dust, etc. becomes in time putrid; though in well-stopt bottles it remains unaltered; I have seen some, which after keeping for many years was perfectly clear and tasteless. Distilled water suf-fers no alteration in either circumstance. The saline matter of snow water is commonly of the nitrous kind, composed of the acid of nitre united with calcareous earth.

The next in purity is rain water, collected with the same precautions as the foregoing, after the rain has continued for some time, so as to clear the air from infects or other light bodies that may float in it. Neither this water nor the preceding discover any heterogeneity on the common trials with acids, alkalies, soap, blue vegetable juices, or metallic solutions, till great part of the aqueous fluid has been sepa-rated by evaporation. Evaporated to dryness, it leaves four or five grains of solid matter on the gallon. Its salt is often nitrous, and its earth in great part calcareous.

The water of limpid rivers stands next in purity; and proves, though not equally with the two preceding, yet sufficiently soft, and fit for all the purposes of life. Rivers are for the most part purer and softer than the springs from which they are supplied; at a distance from, than near to the source; when their course is rapid, than when flow.

Of spring waters, there are some, which approach in purity to that of rain; but the greater number are of all waters the hardest and most impure. Some, even of those which the eye and palate judge to be good waters, contain above an hundred grains of solid matter on the gallon. The saline part of these waters is most commonly nitrous (a) or muriatic, that is, composed of the nitrous or marine acids united with earths: on adding to them, by little and little, a solution of any alkaline salt, the liquor becomes turbid and milky, more and more, till the acid, completely neutralized by the alkali, parts with all the earth, which on (landing fettles to the bottom. The water thus corrected, though really no purer than at first, is found perfectly soft for oeconomical uses, and much less, if at all, detrimental to health; its pungent, austere, earthy salt, being now con-verted into a mild neutral one.

River waters generally putrefy sooner than those of springs: during the putrefaction, they throw off a part of their heterogeneous matter, and at length become sweet again and purer than at first. Hard waters are remarkably indisposed to corrupt, and even preserve putres-cible substances for a considerable length of time: hence, as Dr. Home observes, they seem to be best fitted for keeping at sea, espe-cially as they are so easily softened by a little alkaline salt.

(a) Home, experiments on bleaching. Marggraf, mem. de l'acad. des sciene. de Berlin, anno 1751.

The purest waters soonest freeze; hence ice is purer than the water that remains unfrozen: on this principle, vinous and some saline liquors may be freed from a part of their superfluous water by gentle congelation. Ice, exposed to the open air, loses of its weight, its superficial parts being dissolved or abraded by the motion of the atmosphere. This property of ice was known to Hippocrates; who, imagining not the ice in its whole substance, but some of its finer and lighter parts to be dissipated, was hence led unjustly to condemn both melted ice and snow as the mod impure of all waters.

With regard to the medicinal powers of pure water, little more can be said, than what is too obvious to require being mentioned. Simple fluidity; universal innocence, or the absence of every quality that can offend the tenderest organ; miscibility with all the animal juices, in a state of perfect health, except fat; unfitness to dilute or mingle with them when greatly thickened, as in some diseases; a dis-position to pass off by the cutaneous pores, more speedily and more plentifully than by the kidneys, in consequence perhaps of its total want of irritation; make the principal part of its medical character. To which may be added, that it is the most commodious medium for applying to the human body the powerful agents, heat and cold; of which the one expands and relaxes, the other contracts and con-stringes, all the fluid and soft parts of the animal machine.