HYGIENE 567 regards health, so long as reasonable care is observed. In a healthy body an adaptation to circumstances rapidly takes place, and an equilibrium is soon established. Thus, it used to be supposed that great heat increased the temperature of the body, but later observations have shown this to be erroneous, and that the balance is soon re-established by the process of transpiration; should this, however, be arrested, then a rise of temperature may take place, and disease of a febrile character be established. 2. The soil or site of the dwelling is, however, of greater moment, and much that has been attributed to climate has been more truly due to locality. Soils are generally divided into moist and dry, permeable and impermeable, and again subdivided according to formation, composition, slope, &c. Healthy soils are those which are dry and permeable, or which have such a slope as renders drainage easy ; on the other hand, soils which are flat, moist, and impermeable are generally unhealthy. Soils containing much organic matter are to be avoided, such as alluvial soils generally, as well as all marshy districts. The air in soils is generally more or less impure, hence the unadvisability of occupying dwellings below the ground level or situated immediately on its surface. The water in the soil is a question of great importance, apart from the mere moisture. At varying distances from the surface, but everywhere, there exists a great subterranean lake or sea, known as the groimd-water or water-table, which is constantly in motion, both vertically and horizontally. Its horizontal movement is towards the nearest water-course or towards the sea ; its vertical move ment is determined by rainfall chiefly. Much importance has been attached to it, and the following points may be con- sideredas accepted by most hygienists : (1) a permanently high ground-water, that is, within 5 feet of the surface, is bad, while a permanently low ground-water, that is, more than 15 feet from the surface, is good ; and (2) violent fluctuations are bad, even with an average low ground- water; a comparatively high ground-water with moderate and slow fluctuations may be healthy. According to the school of Pettenkofer, it is the ground-water which determines the spread of certain forms of disease, such as cholera and enteric fever. A previously high level, succeeded by a fall, with a certain height of temperature in the soil-air, is the condition believed by them to be the one most favourable for disease production. Healthy soils are the granites, metamorphic rocks, clay-slate, limestone, sandstone, chalk, gravel, and sand ; unhealthy are clay, sand and gravel with clay subsoil, alluvial soil, and marsh-lands, with the exception of peatiands. Among the unhealthy soils ought also to be included all " made " soils, particularly those that are formed so often in towns from rubbish of all sorts. Such soils ought not to be occupied as building sites for at least two years. 3. The sanitation of dwellings involves numerous points. The site has been considered in the previous section, but the importance of excluding soil emanations must be insisted upon. The placing of a dwelling in any spot of ground tends to exert an extractive force upon the soil, because the air of the dwelling is almost always warmer than the external air, and there is therefore a constant danger of sucking up the more or less impure soil-air into the dwell ing. Not only is this a recognized source of disease, but fatal cases of direct poisoning have sometimes resulted, as when coal-gas has -escaped into the soil below or near a dwelling. An impervious foundation is therefore necessary, although this precaution is too often neglected, even in high-class dwellings. Houses ought to be so arranged that they may receive plenty of light, not merely for work or convenience, but as a matter of health. Sunlight, for full health, is as necessary as air, and this is now eo strongly recognized in America that in many of the hospitals in that country rooms are provided where patients may take a " sun-bath." The materials of which houses are built are various. Wooden dwellings have advantages, but there is always the danger of fire. Brick or stone is most commonly used, but very good dwellings may be made of concrete or even of mud. Probably the best material is good, sound, well- burnt brick. Dryness must be secured by means of damp- proof courses along the foundations, hollow walk, and cementing or slate-hanging externally. Non-absorbent surfaces internally are important, although some writers, such as Pettenkofer, &c., have been inclined to attribute the unhealthiness of dwellings to the impermeability of the walls obstructing air change. But where air can pass, organic matter can lodge and become 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 ; if the latter is used it had better be glazed. 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, caulked, and oiled or varnished so as to make them water-tight. Proper cubic space is a matter of great importance, for upon it depends the renewal of air. The air of an air-space can seldom be changed of tener than three times an hour, hence the space ought to be large enough to allow of such rate of change providing enough of air for respiratory purposes. The furniture of rooms, especially sleeping rooms, ought not to be too massive ; whilst curtains and hangings too often form traps for dust and organic matter. The warming of houses is important, and is generally badly and wastefully done. The open fire-place has great advantages, but it is in many cases insufficient. Where any general system is employed it is better to warm the air in the room itself, as by pipes conveying hot water or steam, than to warm it before delivery. Overheated rooms are a source of ill-health. For sitting-rooms 60 to 65 is quite enough ; for a study or work-room 60 is sufficient, even in some cases less than this. A sleeping-room need never be above 60, often with advantage below it. Fresh air ought not to be sacrificed to temperature, except under extreme circumstances. Dwellings should not be occupied for some time after building, till they are thoroughly dry. Rheumatism, chest diseases, &c., are very apt to arise from neglect of this precaution. Scrupulous attention to cleanliness is necessary in dwellings, and there is wisdom in their periodical vacation for a certain time, so as to let them lie fallow, as it were, and interrupt the continuity of deposit of organic matter. Dwellings ought to be scattered over as wide an area as possible, for statistics show that sickness and death-rate are often inversely proportional to the amount of area per head occupied by a community. The area per head in London is estimated at double that of Paris and many other cities, whilst at the same time its death-rate is smaller than that of any other large city in Europe. 4. Air is the prime necessity of life. Food or water may be abstained from for a considerable time, and we may thus have an opportunity of replacing either should we doubt its purity or wholesomeness, but the atmosphere around us we must breathe or die. Hence the paramount necessity for having it pure. But, although this is apparently so obvious, attention to its importance has been very generally omitted. Air consists of a mechanical mixture of oxygen and nitrogen, in the proportion of nearly 21 per cent, of the former to 79 of the latter, with small quantities
in addition of carbonic acid, moisture, organic matter,
