ganic matter from the surface; a soil which has a low ground-water level, and which retains but little dampness; a soil which admits of free circulation of atmospheric air with the ground air; a soil that does not admit of collections of standing water, and that has slope enough to insure drainage. Where soil cannot be selected paving and tree-planting correct many evils. Paving prevents the diffusion of ground air and the entrance of sewage or contaminated rainwater. Trees absorb carbonic acid gas and moisture and yield oxygen, which in turn assists chemical conversion of organic matter. Cementing of cellars and laying damp-proof material upon foundations before erecting walls are also protective measures against dampness and pollution. In wet localities or in settlements necessarily built for commercial reasons near marshy land, through subsoil drainage by means of trenches or drain-tile, the level of the ground water may be lowered to a safe position. See Drainage.
Air. Air is an imperfect gas consisting of 79 per cent. of nitrogen and nearly 21 per cent. of oxygen, together with small quantities of carbonic acid, ammonia, watery vapor, and impurities. We may neglect the consideration of the small quantities of helium, neon, argon, krypton, and xenon, the rare gases found in the atmosphere during recent chemical investigations. Air is the prime requisite for existence, and upon its purity depends to a large extent the growth, development, and health of animal life. Saturation of the atmosphere with water is called 100 per cent. of humidity. Average health demands a humidity of from 65 to 75 per cent., the lowest amount of aqueous vapor in the air being 35 per cent. Impurities in the air are from various sources. Air is vitiated by respiration, combustion of fuel or of illuminating gas, decaying vegetable or animal matter, and by gases arising from manufacturing and various occupations. Expired air contains 100 times more carbonic acid and nearly 5 per cent. less oxygen than ordinary atmospheric air. Emitting with each expiration 22 cubic inches of air and respiring 18 times a minute, each adult emits 570,240 cubic inches, or 330 cubic feet of air in 24 hours. In this total there are 14.52 cubic feet of carbon dioxide. Physical activity increases this total. Combustion of fuel and gas adds carbon monoxide and dioxide, smoke, and soot to the atmosphere. Factories, etc., add dust, chemical vapors, and volatile substances to the air. Small amounts of impurity do a little damage to health, large amounts undermine it. Hence ventilation becomes necessary, that is, comparatively pure air must be substituted in dwellings for vitiated atmosphere. See Heating and Ventilation.
Water. The atmosphere is the source of water supply. The vapor of water therein is condensed and falls in the form of rain, snow, or dew. Rain, obviously, must carry down with it the impurities in the atmosphere—gases, dust, and bacteria. It must cause deeper deposition of organic matter as it passes into the soil. It becomes either surface water augmenting the streams, or ground water supplying wells and subterranean reservoirs. Impure water carries the germs of many diseases, as typhoid fever, diphtheria, diarrhoea, dysentery, malaria, cholera, probably yellow fever, etc. The pollution of surface water by the entrance of sewage and of decomposing organic matter is very easy and is a prevalent cause of disease. See Filter and Filtration; also Water Purification; Water Supply; Water-Works.
Dwellings. Besides the site of a dwelling and the desirability of its freedom from dampness and ground air, to which attention has already been given, a house for living or for business purposes should give access to an abundance of sunlight. The heat rays, luminous rays, and actinic rays of light all effect decomposition of organic material and hasten reconstruction processes. The materials of which houses are built are various. Wooden dwellings are common in country localities, but they are always open to the objection of the greater danger of fire. In cities brick or stone is most commonly used, but very good dwellings may be made of concrete. Probably the best material is good, sound, well-burnt brick. Dryness must be secured by means of damp-proof courses along the foundations and hollow walls, and cementing externally. Non-absorbent surfaces internally are important, although some have been inclined to attribute the unhealthfulness of dwellings to the impermeability of the walls obstructing air change. But where air can pass organic matter can lodge and becomes a source of danger. It is better, therefore, to have non-absorbent surfaces as much as possible, and to provide for ventilation in other ways. Paint that can be washed is therefore better than paper. Care should be taken to scrape off all old papers beneath, as they and the paste used with them tend to decompose and become injurious to health. Ceilings ought to be impervious as well as walls, and floors ought to be made of well-fitting seasoned wood, calked and oiled or varnished so as to make them watertight. The proper cubic space has been stated. Arrangements should be made for change of air once in three hours, if conditions of constant change do not exist. The furniture of rooms, especially sleeping rooms, ought not to be too massive; white curtains and hangings too often form traps for dust and organic matter. The warming of houses is of exceeding importance. See Heating and Ventilation.
Scrupulous attention to cleanliness is necessary in dwellings, and there is wisdom in the use of rugs or loose carpets which may be removed daily from rooms and thoroughly cleaned. Corners should be thoroughly freed from dust as well as nooks underneath and behind large pieces of furniture, spaces above rows of books, the wall sides of pictures, etc., for dust forms a well-adapted nidus for disease germs, especially of the bacteria which produce suppuration. Closely allied to the ordinary cleaning of the interior of dwellings is the problem of the removal of excreta, waste, and garbage. Practically waste consists of: (1) Garbage, including kitchen refuse, offal, bones, etc.; (2) refuse, including paper, dust, ashes, clothing, carpet, broken furniture, iron and other waste metal, as well as ‘trade refuse,’ which includes excelsior, straw, wood shavings, leather scraps, tobacco stalks, felt cuttings, tin scraps, etc.; and (3) sewage, including animal excrement (fecal and urinary), wash water from bathing, laundering clothes, washing culinary utensils, cleaning house, etc. Properly separated, ashes and dust are useful in filling sunken lots, marshes, etc. Paper, metal, and most trade refuse have a market value. Sewage and garbage are valuable fer-
