_Purifying Action of Soils._—The disinfecting action of dry soil, absorbing offensive gases from manure piles and from earth closets, has already been alluded to. Similarly it is a matter of common experience that the colored and otherwise offensive drainage from manure piles, tanneries, dyeworks, etc., is not only deodorized but also decolorized when passed through a sufficiently thick layer of clay soil. The filtration through fine sand by which the drinking waters of cities are so commonly purified before delivery to the consumer are familiar examples of the same effects. Equally familiar, however, is the fact that this power of decolorization and retention of offensive compounds is limited; that after a while the filtering earth or sand becomes saturated, and afterwards the water or drainage will pass through without any sensible purification. It is therefore clear that this purifying effect of earth cannot be relied upon for the permanent protection of wells from the surface-drainage from barnyard or house refuse. Even if fissures or layers of sand or gravel should not intervene so as to permit of the direct communication of surface-drainage with wells, it is certain that in the course of a few years at most, the intervening earth will become so far saturated with the noxious ingredients that the latter will pass through unhindered, and may contaminate to a considerable extent the domestic supply of drinking water. _Waste of Fertilizers._—The same, of course, holds true in regard to manure-water, or soluble fertilizers of any kind used on the soil of a field. The soil will retain them to a certain extent; but beyond that limit any surplus added will be quickly washed through into the country drainage by the rains. Moreover, a soil once so saturated will yield to rain water filtering through it, notable amounts of all the ingredients absorbed in it; and, at least so far as the physically condensed soluble ingredients are concerned, long-continued leaching with pure water will inevitably result in the withdrawal of additional amounts of absorbed ingredients, apparently dividing themselves up _pro rata_ between the water and the soil. It is obviously of the utmost importance to the farmer to know to what extent the soil will retain manurial ingredients against the influence of leaching rains; for unless this is taken into consideration, it may readily happen that the fertilizer supplied before a rainy season will be washed through beyond the reach of plant-roots, and so practically become a dead loss.