Every person who tills the soil is acquainted with the remarkable transforming power of the superficial layers of the earth upon manure and excrement. Out of these offensive wastes harmless substances are produced which are essential to the growth of vegetation. This power, known as decay, is now generally attributed to very minute organisms (bacteria) which are found in immense numbers in the superficial layers of the soil, which diminish in number as we go deeper, and which completely disappear below a depth of 6 to 12 feet, according to the physical condition of the soil. Bacteria are more numerous where waste and excrement are most abundant. When night soil and manure are deposited in excavations or so-called cesspools in the earth, from which the fluid matter may enter the ground at some depth below the surface, where the air or certain kinds of bacteria can penetrate only to a slight extent, the substances, which under the influence of the air (oxygen) and of bacteria near the surface, would have decayed, now undergo partial putrefaction with the setting free of disagreeable gases and odors. The deeper layers of the earth slowly become saturated with organic matter, which is carried by the ground-water into the wells or springs near by. There is also some reason to believe that disease germs live longer in the oxygen-free depths of the soil than at or near the surface. The extent to which the filling up of the soil with excrementitious matter may go on in densely populated cities has been shown by Fodor for the Hungarian city Budapest. By analyzing the soil at different levels from the surface to a depth of about 13 feet, he found, over an area comprising 15 acres, about 1,000,000,000 pounds organic matter, equivalent to the excrement of 100,000 people voided during thirty-seven years. [Illustration: Fig. 1.--The shallow barnyard well, with privy vault and manure heaps near by. The water is likely to receive fluid from these at any time.] To the surface of the earth we owe thus a purifying influence whose activity furnishes us vegetation and food on the one hand and preservation from disease on the other. This purifying power is not possessed by the deeper layers, and therefore the percolation of organic refuse into them from deep cesspools is wasteful to agriculture and dangerous to our storehouse of drinking water. Even the surface of the soil when overloaded with sewage loses partially its power of purifying the organic matter. After sufficient rest, such an overloaded soil regains its original power. The purifying activity of the soil from a sanitary aspect is the same as that governing fertility from an agricultural standpoint, hence any further discussion of this subject is unnecessary.