There are many instances on record where troops have sickened and many died of putrid fever and dysentery, from drinking the water of stagnant pools and ditches or of rivers, as of the river Lee, near Cork, (Ireland,) which, in

passing through the city, receives the contents of the sewers from the houses, and is otherwise unwholesome.

The organic matter contained in river water consists chiefly of the exuviaj 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 f'g-ths 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