The most obvious method of removing gross materials from water is by filtration. All solid particles - silts, sand, clay, and other mechanical impurities - may be removed in this way. In water taken from running streams the most abundant mechanical impurity, especially after heavy rains, is silt or clay. In many sources of water supply these impurities are never absent. As an example, it may be said that the waters of the Missouri River are never clear, and the Ohio River is probably muddy three-fourths of the year. Many large cities on the banks of these streams take their water supply from the Rivers. This is true also - or was true - of the Potomac, the waters of which are muddy for a large portion of the year. Until the installation of the filtration plant at Washington it was a rare thing to have clear water unless the river waters were first collected in reservoirs and allowed to settle for a long time. Two principal methods of filtration are practised: the first may be described as the chemical method. In this process the suspended matter is coagulated by the addition of alum, ferrous sulphate or lime, or all three combined. These bodies usually produce the coagulation of the suspended silt, by the process technically known as flocculation. By some the suspension of the silt in water is supposed in one sense to be an electrical phenomenon, and the addition of an electrolyte like some of the soluble salts mentioned above is sufficient to produce coagulation and precipitation. Whatever be the cause, however, the fact remains that to remove suspended silt from water by any kind of filtration coagulation can be effected as described. It is extremely difficult to remove suspended silf from water by any kind of filtration except that of the sand bed, to be described hereafter, without previous coagulation. After the coagulation the particles of coagulated material can be separated by passing through filters usually made of sand or sand and charcoal, which retain the precipitated matters. These filters, as in fact all others, require frequent washing in order to remove the occluded silt. The filtration may be accomplished by gravity, that is by throwing the water on the filtering bodies and allowing it to percolate, or by mechanical force, that is by forcing the water through the filters by pumps. For small supplies, such as for factory use or for small cities, the filtration by means of pumps and small filters can be practised. For large supplies, however, more economical methods are required.