The organic matter contained in river water consists chiefly of the exuviæ of animal and vegetable substances, but another class of impurities consists of living beings, (animals and vegetables.) The aquatic animals, which have, from time to time, been exhibited in this city by means of the solar microscope, are collected in stagnant pools, and are not found in river or well water. The quantity of organic matter contained in the Croton must be extremely small, as this, together with the silex, iron, and magnesia, amount to only 4/10ths of one grain to the gallon. WELL WATER,--or _pump_ water, as it is often called in cities, is essentially the same as spring water, but liable to impregnation, owing to the land springs filtering through the walls, and conveying impurities into it. This is sometimes prevented by lining them with cast-iron cylinders, or by bricks laid in water-cement. Dr. Percival affirms, that bricks harden the softest water, and give it an aluminous impregnation. The old wells must, therefore, furnish much purer water than the more recent, as the soluble particles are gradually washed away. It contains a greater proportion of earthy salts, and of air, and has a greater specific gravity than other spring waters. Owing to the fact, that it contains a larger quantity of bicarbonate and sulphate of lime, than river water, it decomposes and curdles soap, and is then denominated _hard water_, to distinguish it from those waters which mix with soap, and are therefore called _soft waters_. The reason that hard water does not form a pure opaline solution with soap, is, because the lime of the calcareous salts, chiefly the _sulphate_, forms an insoluble compound with the margaric and oleic acids of the soap. Here a double decomposition ensues, the sulphuric acid unites with the alkali of the soap, setting free the fatty acids, which unite with the lime to form an insoluble earthy soap. Hard water is a less perfect solvent of organic matter than soft water; hence in the preparation of infusions and decoctions, and for many economical purposes, as making tea and coffee, and brewing, it is much inferior to soft water, and for the same reasons it is improper as a drink in dyspeptic affections, causing irritation, and a sensation of weight in the stomach. The abundance of this earthy salt in the water of Paris, and London, of many parts of Switzerland and this country, cause uncomfortable feelings in strangers who visit these places. It is also said to produce calculous complaints in the inhabitants, a result which might be expected, owing to the low solvent power of the water not being sufficient to carry off the animal acid, which concretes in the kidneys to form calculi.[11] Well water can be easily freed from these earthy salts; boiling precipitates the carbonate of lime by driving off the carbonic acid which holds it in solution; and the addition of a little carbonate of soda precipitates the lime, if any exist in the water.