Mere exposure to the air, even if accompanied by violent agitation, is comparatively powerless for the removal of polluting organic matter from water. Although, however, the flow of a river has thus but little effect in purifying the water by the oxidation of the dissolved organic matters, it has a most material influence in the removal by subsidence of a large proportion of the suspended impurities both organic and mineral, especially if the flow be sluggish in places. In passing through still pools, the turbid streams let fall its load of grosser mechanically suspended particles, and thus the water becomes clearer, although the dissolved impurity remains nearly as great as ever. It is, doubtless, this clarification by subsidence which has led to the very general but erroneous belief in the rapid self-purifying power of running water. RESULTS OF SUBSIDENCE. ===============================+=============================== | SUBSIDENCE FROM 100,000 PARTS. RIVERS +----------+----------+--------- | MINERAL | ORGANIC | TOTAL | MATTER. | MATTER. | SOLIDS. -------------------------------+----------+----------+--------- Irwell, after flow of 11 miles | .88 | .48 | 1.36 Irwell, after flow of 11 miles | .38 | .84 | 1.22 Mersey, after flow of 13 miles | .10 | .04 | .14 Darwin, after flow of 13 miles | .54 | 1.42 | 1.96 -------------------------------+----------+----------+--------- RESULTS OF SUBSIDENCE. ===============================+=============================== | PER CENT OF REDUCTION | OF MATTER IN SUSPENSION. RIVERS +----------+----------+--------- | MINERAL | ORGANIC | TOTAL | MATTER. | MATTER. | SOLIDS. -------------------------------+----------+----------+--------- Irwell, after flow of 11 miles | 47.8 | 50. | 48.6 Irwell, after flow of 11 miles | 14.3 | 30.9 | 22.7 Mersey, after flow of 13 miles | 10.6 | 13.3 | 11.3 Darwin, after flow of 13 miles | 30.3 | 79.8 | 55.1 -------------------------------+----------+----------+--------- AERATION is the destruction of animate life by oxidation, and is best accomplished by placing weirs across streams, sheet flashing, or spreading of water in thin sheets, or by roughness of beds or banks of running waters. The benefits may be ascertained, chemically, by the presence of nitrates and nitrites. The Water Supply Commission of Engineers, for the investigation of the water system of Philadelphia, say: