Illustrations of the Croton Aqueduct

by F. B. Tower

APPENDIX.

BY CHARLES A. LEE, M. D.

WATER.

(Chiefly compiled from the works of Thomson, Pereira, Whewell and others.)

Water was regarded by the ancients as an elementary substance, and as a constituent of most other bodies. This opinion was somewhat modified by the experiments of Van Helmont and Mr. Boyle, who maintained that it could be changed into all vegetable substances, as well as into earth; but it was substantially held until the middle of the last century, (1781,) when Mr. Cavendish proved that this liquid was a compound of oxygen and hydrogen.

Natural History. In the inorganized kingdom.

Water is very generally diffused over the surface of the globe, forming seas, lakes, and rivers; it is mechanically disseminated among rocks, constitutes an essential part of some minerals, and always exists to a greater or less extent, in the atmosphere. In the air, water is formed in two states; as a vapor (which makes about one-seventieth by volume, or one one-hundredth by weight of the atmosphere) it is supposed to be the cause of the blue color to the sky; and in a vesicular form, in which state it constitutes the clouds. Terrestrial water forms about three-fourths of the surface of the terraqueous globe. The average depth of the ocean is calculated at between two and three miles. Now as the height of dry land above the surface of the sea is less than two miles, it is evident, that if the present dry land were distributed over the bottom of the ocean, the surface of the globe would present a mass of waters a mile in depth. On the supposition that the mean depth of the sea is not greater than the fourth part of a mile, the solid contents of the ocean would be 32,058,939 cubic miles (Thomson’s Chemistry.) The quantity of water mechanically disseminated through rocks, which serve merely as a natural reservoir for the time, must be, in the aggregate, very considerable, though it is impossible to form any very accurate estimate of it. Even in those rocks which merely supply springs, the amount of disseminated water must be enormous; for they so far resemble filters, that are necessarily charged with the fluid before they permit it to pass out. De La Beche has advanced the opinion that capillary attraction has great power, both in mechanically disseminating water among rocks, and in retaining it in them when so disseminated, and that it therefore keeps them, to a certain extent, saturated with moisture, and assists in promoting a more equal flow of water in springs. Capillary attraction and gravity probably carry water down far beyond those situations where it can be returned in springs, at least cold springs, for there are certain circumstances connected with those which are thermal, which go to prove, that the water thrown up by them may have percolated to considerable depths. It is very evident that most rocks contain disseminated moisture, for there are few which, when exposed to heat, do not give water. Sulphate of lime, for example, or plaster of paris, contains about 20 per cent., and common serpentine, as much as 15 per cent. of it. Soap-stone has 4 per cent., and even quartz 2 per cent. of water, in their composition. This fluid exists in minerals either as water of crystallization, or combined as a hydrate.

But though water is thus generally diffused over the surface of the globe, yet it is not found perfectly pure in any place; even the rain and the snow that descend from the clouds, the condensation, as it were, of a natural distillation, are slightly tainted by saline matters; which circumstance can only arise from the great solvent power of water enabling it to take up a portion of most substances with which it comes into contact, in its natural condition. In many lakes, and in the ocean, the quantity of saline matter is so great as to render it unfit for diluent purposes; but, when sea-water freezes, the saline impregnations are deposited; and the ice affords fresh water. In the state in which water is generally employed as a diluent, its impregnations are in small quantity, and not usually sufficient either to dim its transparency, or to give it color, smell, or taste, and consequently to render it unfit for the ordinary purposes of life. Water, therefore, which is transparent, colorless, inodorous, and tasteless, is called good and pure, and none other can be called such; though some medical writers are of opinion, that it is not necessary it should be in this pure state for common use. Such opinion however is undoubtedly erroneous—

II. In the organized kingdom. Water enters largely into the composition of organic substances. It constitutes, at least, four fifths of the weight of the animal tissues, being the source of their physical properties, extensibility and flexibility. This water is not chemically combined in them: for it is gradually given off by evaporation, and can be extracted at once by strong pressure between blotting-paper. When deprived of its water, animal matter becomes wholly insusceptible of vitality; except in the case of some of the lower animals, which, as well as some plants, revive when again moistened. According to Chevreul, pure water alone can reduce organized substances to this state of softness; although salt water, alcohol, ether, and oil, are also imbibed by dry animal textures. Moist animal tissues, by virtue of their porosity, allow soluble matters, which come into contact with them, to be dissolved by the water which they contain, and which oils their pores: if the matters are already in solution, they are imparted by their solutions to the water of the tissues. Gaseous substances are taken up in the same way. Water exists in nearly as large a proportion in vegetable as in animal substances.

Properties. Pure water, as has already been stated, is a transparent liquid without color, taste, or smell. Some have doubted whether it is entirely inodorous, from the fact that the camel, and some other animals, can scent water to a considerable distance, and also whether it can be called colorless, as all large masses of water have a bluish-green color. This phenomenon is, however, probably owing to the presence of foreign matters. It refracts light powerfully, is a slow conductor of heat, when its internal movements are prevented, and an imperfect conductor of electricity. It is almost incompressible, a pressure equal to 2000 atmospheres occasioning a diminution of only one-ninth of its bulk; or, when submitted to a compressing force equal to 30,000 lbs. on the square inch, 14 volumes of this fluid are condensed into 13 volumes; proving that it is elastic. Water being the substance most easily procured in every part of the earth in a state of purity, it has been chosen by universal consent, to represent the unit of the specific gravity of all solid and liquid bodies. A cubic inch of water at 60° Fah. weighs 255.5 grains; so that this fluid is about 815 times heavier than atmospheric air, but being the standard to which the weight of all other substances is referred, its specific weight is said to be 1. Accordingly when we say that the specific gravity of a body is two we mean that it weighs twice as much as the same volume of water would do. Water unites with both acids and bases, but without destroying their acid or basic properties. Thus the crystallized vegetable acids, tartaric, citric, and oxalic, are atomic combinations of water with acids. Caustic potash (potassa fusa) and slaked lime may be instanced as compounds of water, and basic substances; these are therefore called hydrates. The crystallized salts, such as alum, common salt, sulphate of soda, sulphate of magnesia, borate of soda, (borax,) &c., contain a large amount of water as a chemical constituent, called water of crystallization. Water rapidly absorbs some gases, as ammonia, fluoride of boron, &c., but it is neither combustible, nor, under ordinary circumstances, a supporter of combustion.