The ability of water both to dissolve matters with which it comes in contact and to carry them off mechanically, has already been referred to. In this sense water is the universal scavenger: it flushes the streets; it carries off the refuse of factories, of dwelling houses, and of stables; it transports mechanically large quantities of mineral and organic matter, and dissolves in the earth through which it passes all the soluble constituents of the soil. The water, therefore, when it reaches the spring, the stream, and the river, is in one sense Nature's sewage, and carries either in solution or in suspension all of the refuse of Nature's activity to which it can gain access. In this country the lower Mississippi River represents the greatest sewer of the world. It drains the entire area between the crest of the Alleghanies and the crest of the Rocky Mountains. This area is in round numbers 1,260,000 square miles. The city population on the banks of the Mississippi and the streams tributary thereto is about 10,000,000. The rural and town population is probably double that number. It may be said that fully 40,000,000 people, or nearly half of the population of the United States, reside in the Mississippi Valley. The river is the cloacum magnum of the United States. The character of the water which flows past the City is a matter of great interest. A study of the Mississippi water was carefully made by a committee appointed for the purpose of proposing a system of water supply and sewerage for the City. The results of a long series of studies extending over three years show that the Mississippi River is a very muddy stream containing about 600 parts per million suspended matter, consisting chiefly of silt and clay. If this suspended matter were separated from the water it would amount to nearly three tons of solid matter in each million gallons of water. During very dry seasons the quantity of suspended matter is very much diminished, but during periods of flood there has been found to be as much as 2,400 parts per million.