=Nitrification= is effected by micro-organisms in the soil. Ordinary garden mould and agricultural humus contain large numbers of micro-organisms. Their number diminishes with the depth of the soil, and below 12 to 15 feet there are few. Apart from the occasional presence of pathogenic (disease-producing) micro-organisms, the most important are those producing oxidation of organic matter, especially _nitrification_. This occurs at a less depth than 4 feet from the surface of the ground. The operation of these micro-organisms is necessary to convert sewage and other impurities into harmless nitrites and nitrates, and it is regularly going on in all normal soils. That the power of purification of sewage by soil is due to the micro-organisms in the latter, can be proved by the fact that when the soil is baked, it loses for a time its purifying power. =The Air contained in the Soil= varies greatly in amount with the character of the soil, and with the level of the ground-water. As the ground-water rises, the ground-air is driven out. Thus, after a heavy rainfall a large proportion of this air will be displaced. Variations in barometric pressure, and a rise or fall of temperature, cause movements in ground-air. A house artificially warmed is liable to receive air from underground, unless means are adopted to make the floors impervious. The warmth of the house acts as an air-pump, aspirating the colder air into its interior. The air from cesspools or defective drains may be similarly aspirated into the house; and the same cause particularly explains the unhealthiness of houses built on “made soils”. Coal gas has occasionally made its way into houses when not laid on to them, by the gas escaping from leaky pipes in the street often following the track of water or drain-pipes until it is aspirated from beneath the house into its interior. This has resulted in one instance in an explosion, and in others in poisoning by the gas. [Illustration: FIG. 42.] The occurrence of currents of air in soil may be illustrated by a simple experiment. In Fig. 42 B is filled with fine sand in which is imbedded the tube A with its open end F at the bottom of the sand C. The upper end of A is connected by the rubber tubing D with the U-shaped tube E, in which is inserted some coloured water. When the experimenter blows on the surface of the sand at A, the impulse passes through the sand up the tube from F, and deflects the water in the syphon bend at E. The _amount_ of ground-air varies greatly. Loose sands often contain 40 to 50 per cent., soft sandstone 20 to 40 per cent., and loose surface-soil many times its own volume. The _nature_ of the air is not accurately known. It is, however, extremely rich in carbonic acid, of which it contains from 1 to 10 per cent. or even more. The carbonic acid is derived from the organic matter in the soil, by the action of bacteria, in a manner analogous to nitrification.