The one most commonly employed is the ploughed and tongued floor (Fig. 41). In this, both edges of the floor are grooved 부천오피 so as to receive strips or tongues of iron or wood, an equal half of each strip being in the groove of each of two boards when they are in place. A less expensive method than the above is to splay the ends of the boards so that they slightly overlap each other. This is not so efficient as the above, but is much better than simply placing the boards side to side as is commonly done.

Solid wood floors resting on a bed of concrete are free from the risk of harbouring dust, and are relatively fire-proof.

Oak or teak in narrow boards, made with close joints, and then oiled and beeswaxed and rubbed to a polish, makes a good and almost non-absorptive floor. One of the best floors is made of concrete, with iron joists, and oak boards laid above this.

218

Carpets are commonly made to cover the entire floor of rooms. This cannot be too much deprecated. Carpets, like curtains, are mere dirt-traps, which become loaded with filth of every description. This is abundantly proved when a carpet is swept, and the dust allowed to settle on all the articles in the room. Such dust, if examined, will be found to consist not only of mineral matter, but also of every description of vegetable and animal impurities. The inhalation of such dust, which may contain particles of fæcal matter, as well as the dried expectoration from consumptive or other infectious patients, is a not infrequent cause of infection to healthy persons.

The substitution of a central carpet, for one covering the entire floor, is a great improvement.

The carpet should be easily removable, in order that it and the floor may be thoroughly cleaned at intervals.

In bedrooms, the less carpet the better. Good Chinese or Indian matting is serviceable, as it does not retain the dust and other impurities which are apt to become fixed in the woolly texture of the carpet. Oil-cloth, linoleum, and similar materials are in common use for covering halls, passages, etc. They are particularly useful in preventing dust from gaining access to the spaces between floor-boards.

The prevention of dust should be the great aim of the householder, as dirt frequently carries infection. Sweeping as ordinarily done scatters dirt over the room, and dusting with a dry cloth fails to remove it. Mechanical sweepers, in which the dirt is collected in a box are valuable. The best plan is to have movable carpets, roll them up for shaking or beating at a distance from any house, and wipe the boards with damp cloths. All wooden and leather furniture, picture frames, etc., should be wiped down with cloths rung out of water so as to be just damp.

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CHAPTER XXXIII.
THE SOIL.
The Varieties of Soil.—The following facts summarise what is regarded as the relative healthiness of various sites for dwellings. The differences between different sites may, however, be reduced to a minimum by having the dwelling well above the ground-level and by protecting it from dampness.

1. Granitic, Metamorphic, and Trap Rocks usually form healthy sites for houses. The slope is generally great, and the ground consequently dry.

2. Clay Slate resembles the last in its effects on health. Water is, however, often scarce, owing to the impermeability of the rocks, and for the same reason occasional floods occur.

3. Limestone and Magnesian Limestone Rocks resemble the last in possessing considerable slope, so that the water passes away quickly. The hard oolite is the best formation under this head, and magnesian limestone the worst.

4. Chalk is a healthy soil when unmixed with clay, and permeable. Goitre is not so common as in limestone districts. If the chalk be mixed with clay, it is often damp and cold.

5. The Sandstones are healthy, soil and air being dry. 부천오피 If mixed with clay, or if clay lie under a shallow layer of sand-rock, the site may be damp. The hard millstone grit is a healthy formation.

6. Gravels of any depth are healthy, unless they are water-logged, as near rivers. Then a house on impervious clay may be drier than one on gravel.

7. Sands are healthy when of considerable depth; they may be unhealthy when shallow, and lying on a clay basis; or when the ground water rises through them from ground at a higher level.

8. Clay, Dense Marls, and Alluvial Soils generally, are apt to be cold and damp. Water is retained in them, and is often very impure. Thorough drainage improves a clay soil, and a house on a clay soil may be so constructed, as not to be damp.

9. Cultivated Soils are not necessarily unhealthy; but

10. Made Soils are always to be carefully avoided, as sites for houses. The materials with which inequalities have been filled up are commonly the contents of dust-bins, or some other refuse. The gradual putrefaction of organic matters renders the air about the houses impure. Such soils require free subsoil drainage, in order220 to keep them dry. It appears that the organic matters in soil are gradually removed by oxidation and bacterial purification. At least three years should be allowed before any such site is built on.

The following table places different geological formations in their order of healthiness for the purposes of a site (Parkes):—

PERMEABILITY
OF WATER. EMANATIONS
INTO AIR.
1. Primitive rocks, clay slate, millstone grit Slight. None.
2. Gravel and loose sands, with permeable subsoils Great. Slight.
3. Sandstones Variable. Slight.
4. Limestones Moderate. —
5. Sands with impermeable subsoils Arrested by subsoils. Considerable.
6. Clays, marls, alluvial soils Slight. Considerable.
7. Marshes, when not peaty Slight. Considerable.
The general geological conditions have an important bearing on the choice of a site for a house in so far as they affect the local climate, and the difficulty of keeping the house warm and dry. Pettenkofer expressed this in his dictum, that we take holiday for change of soil, rather than for change of air. The character of a soil has an important influence on humidity, radiation, evaporation, and in fact most of the factors going to make up “climate.” The immediate local surroundings of a house (page 201) have an even greater influence on its salubrity than the underlying geological formation.

The soil consists of mineral and organic matters. On the amount and character of the animal and vegetable matters (along with the condition of moisture and aeration), the healthiness of a given soil depends. The presence of vegetable matter, subject to alternate wettings and dryings, and to heat, has until recently been regarded as the condition on which malaria depends; but it is now known that malarial places owe their character to their being favourable to the growth of the larvæ of certain mosquitoes (page 307); and that drainage of the soil cures malaria by removing the ponds in which these develop. The two chief agencies at work to rid the soil of organic impurities, are nitrification and the influence of growing plants. The organic matters become oxidised into ammonia, nitrites, and nitrates, and these are eagerly assimilated by vegetation.

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 all221 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. 부천오피 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.


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 organic222 matter in the soil, by the action of bacteria, in a manner analogous to nitrification.

The Water contained in the Soil is divided into moisture and ground or subsoil-water. When air is present in the soil as well as water, the soil is merely moist. Pettenkofer defines the ground-water as that condition in which all the interstices are filled with water, so that, except in so far as its particles are separated by solid portions of soil, there is a continuous sheet of water.

The Moisture in the soil varies in amount. Open gravel will absorb from 9 to 13 per cent. by weight of water; gravelly surface soil 48 per cent.; light sandy soils from 23 to 36 per cent.; loamy soil 43 per cent.; stiff land and clay soils from 43·3 to 57·6 per cent.; sandy and peaty soils from 61·5 to 80 per cent.; peat 103 per cent. (B. Latham). The moisture being derived from the rainfall on one side, and the ground-water on the other, will vary with the amount of these. Some soils are practically impermeable to water, such as trap or metamorphic rocks, unweathered granite, hard limestone, and dense clay; while others, such as chalk, sand, sandstone, vegetable soils are permeable. Commonly the metamorphic rocks and hard limestones present fissures, which render them pervious. The rainfall which does not penetrate the soil flows into the streams and rivers at once, or is re-evaporated. The amount of percolation of rainfall is estimated by an artificial soil-gauge. Most percolation and least evaporation of rainfall occurs from October to March inclusive. The difference between the percolation and rainfall is the loss caused by evaporation and vegetation.

The Ground-water forms a subterranean sheet of water, which is in constant motion. There is first of all, an irregular rise and fall of the water, according as it receives new additions from the rainfall, or loses a certain amount of its substance by percolation and evaporation; and there is, secondly, a constant movement towards the nearest water-course or the sea. Many towns derive their drinking-water from the ground-water, especially that in the chalk. Thus in Brighton there are no streams; but wells are dug in the South Downs about 150 to 180 feet deep down to the level of the subterranean water. Then long adits are tunnelled, parallel to the coast at or near the level of this water, which is thus intercepted on its way towards the sea, and pumped up to supply the town. In Munich, Pettenkofer reckoned the rate of movement of the ground water towards the outlet as 15 feet daily. It is impeded by impermeability, or a deficient slope of the soil. The roots of trees also greatly impede its flow.

The level of the ground-water is constantly changing (see Fig. 7). The alteration in level may be only a few inches either way, while in some parts of India it is as much as 16 feet. The level is generally lowest in October and November, highest in February and March.

A fall in the level of the ground-water may be due to a dry season, or to improved subsoil drainage. A rise in its level is due to an increase in the rainfall, or some obstruction in the outflow,223 as from a swollen river. The tide may influence the level of the ground-water at a great distance. A sudden alteration in the level of the ground-water is a common cause of floods in mines.

The distance of the ground-water from the surface may be only two or three feet, or several hundred feet, the difference being due to the varying level of the nearest impervious stratum of soil. Its distance below the surface of the soil can easily be measured by ascertaining that of the water of a shallow well in the neighbourhood. It should preferably not be nearer the surface than five or six feet. Sudden changes in the level of the ground-water from inundations render any soil unhealthy, and are even more objectionable than a persistently high level. This is especially true in the case of permeable soils. A sudden rising of ground-water expels the air in the soil, together possibly with particles which may comprise infectious material; it also washes similar impurities out of the subsoil, and carries them into neighbouring wells. Numerous epidemics have been traced to this source.