It appears from the above table, that the amount of impurities contained in the Thames water, exceeds those of the Croton by nearly six fold, and that the quantity of lime, held in solution in the former, surpasses that of the latter, about fifteen times. The Thames water differs also from the Croton, in the circumstance that it contains an appreciable quantity of chloride of sodium, or common salt of which the Croton is entirely free. There are but very few streams to be found, whose waters contain less than 4.16 grains of solid matter to the gallon. The carbonate of lime is held in solution by carbonic acid, forming bicarbonate of lime. By boiling, this acid is expelled, and the carbonate of lime is precipitated on the sides of the vessel, constituting the _fur_ of the tea-kettle, and the _crust_ of boilers. River water always contains a quarter or less quantity of organic matter in suspension or solution. As a general rule, the quantity is too small to produce any decidedly injurious effect, but physicians and medical writers agree in the opinion that water impregnated with it to any great extent must be deleterious. Where the quantity of decomposing matter is too small to produce any immediately obvious effects, it is difficult to procure any decisive evidence of its influence on the system. When the amount is considerable, it causes dysentery and fevers, often of a highly fatal character. In a trial at Nottingham, England, in 1836, it was proved that dysentery of an aggravated form, was caused in cattle by the use of water contaminated with putrescent vegetable matter, produced by the refuse of a starch manufactory. The fish, (perch, pike, roach, dace, &c.,) and frogs in the pond, through which the brook ran, were destroyed, and all the animals which drank of the water became seriously ill, and many of them died with the symptoms of dysentery. It was, moreover, shown, that the animals sometimes refused to drink the water, that the mortality was in proportion to the quantity of starch made at different times; and that subsequently, when the putrescent matter was not allowed to pass into the brook, but was conveyed to a river at some distance, the fish and frogs began to return, and the mortality ceased among the cattle. 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.