The ground air is impure due to the organic matter in the soil. This organic matter is decomposed by microörganisms that feed upon such material and break it up into simpler combinations, carbonic acid, ammonia and water. These processes of fermentation and decay result in a purifying action converting the complex organic matter into products which growing vegetation is capable of assimilating. The presence of oxygen, moisture and warmth is essential to the proper execution of these processes. We can thus see that ample provision has been made for the natural purification and utilization of animal and vegetable matter. Nature provides the oxygen in the ground air, the necessary moisture is derived from the ground water and the temperature of the earth is generally sufficient to supply the needed warmth for this natural process of purification. If the ground water rises too near the surface or if the building site is water-logged, the water may be drawn off by means of a subsoil drainage. This will also prevent fluctuation of ground water, for as this water rises the ground air is forced out and causes the atmospheric air to become damp. This brings about a cooling of the air. The moisture will ascend by capillary attraction into the walls of the houses; in the evaporation from the inner surfaces, heat will be absorbed from the surrounding objects and this will cause the houses to become damp and cold. This drainage may be accomplished by placing unglazed porous tiles with the ends in apposition, but not jointed in the subsoil. The fluctuation of ground water is of little consequence in itself; but we can see that in this way the water in wells may become polluted, impure ground air may be forced into houses, and dampness may thus be produced. Stagnant Water Stagnant water favors the breeding of mosquitoes and the development of certain dis-eases, malaria and cholera for example; hence the necessity for proper drainage. Trees are sometimes planted in such districts and they, together with the subsoil drainage, render the conditions more healthful. The trees and other vegetation in hot climates absorb the water from the ground and then it is evaporated from the leaves and in this way soil is made drier. BUILDING CONSTRUCTION Foundation In the building of a house the foundation must receive just consideration. As the foundation holds the entire structure, such material must be used as to give adequate support to this weight. Care must be exercised in the evacuations that the basement be protected from dampness and that the entire site beneath the foundation be properly drained. The ground water should be ten feet beneath the surface of the site. It is recommended by some hygienists that the basement floor be of cement six inches thick to prevent the entrance of ground air and moisture. This should be covered with a wood floor.