the general vitality of the individuals living in the locality.

Pathogenic Bacteria in Soil.-Some of the pathogenic bacteria are apparently capable of living in the soil for a long time, and some of them may even be able to multiply in the soil. Among these, Bacillus anthracis, Bacillus tetani, and Bacillus cedemæ are perhaps most capable of subsisting in the soil because of their faculty of passing into the spore stage. Bacillus sporogenes of Klein may also be included in this class.

Bacillus typhosus and the cholera organism are less tenacious, and die after a time through the detrimental influence of the soil organisms. Bacillus tuberculosis can remain alive in soil for some time when protected from the influence of soil organisms, though the danger of infection through polluted soil is probably a remote one. The pyogenic cocci and the diphtheria bacillus cannot exist in the soil for any length of time unless protected from the influence of the soil organisms.

Improvement of a Damp Soil.-A damp soil may be improved by opening the outflow or by laying a system of underground drains. The construction of sewers often serves to drain the soil to a considerable extent because the ground-water follows the outside of the

sewer.

Configuration of the Surface and Soil-covering.— Aside from the intrinsic nature of the soil itself, and the character and amount of air and water contained in its interstices, the healthfulness of a soil is influenced also by the configuration of the surface, the condition of the surface, and the nature of the soil-covering. With regard to the configuration of the surface, it may be said that, as a rule, highlands are more healthful than lowlands. The degree of healthfulness of lowlands is influenced by the nature of the soil composing them. The condition of the surface of the soil with regard to soil-covering which is least healthful is what is known as a desert. Here the soil is exceedingly dry and cannot be cultivated.

Cultivated areas and areas covered with forests are more healthful, because the soil is shaded and thus the heating effect of the sun's rays is partly excluded. Cultivated areas that are thickly populated are less healthful because of the organic impurities which gain access to the soil. The soil of cities can be maintained in a comparatively healthful condition only by systematic drainage, so as to carry away all the organic impurities without contaminating the soil, air, or drinking-water.

CHAPTER XVI.

HABITATIONS.

THE first consideration in the selection of a site for a habitation is the nature of the soil with regard to dampness and organic impurity, since these are the principal factors in rendering a soil unhealthful. The house should stand upon a site the subsoil of which is naturally dry, or is properly drained and free from impurity. The configuration of the surface, the elevation, and the exposure are important features in rendering the locality favorable for a healthy habitation. The nature, source, and amount of the water-supply should be investigated. The possibility for the economic and safe disposal of all refuse matter must also be considered. The locality should be sufficiently elevated to secure good drainage away from the house. A southern exposure is preferable, especially in colder climates. The proximity of large bodies of water and of marshy areas have detrimental influences upon the healthfulness of the locality.

The habitation should be so situated with relation to others surrounding it that an abundant supply of fresh air and sunlight can be secured. The healthful influences of sunlight and fresh air cannot be ignored. The absence of sunlight and deficiency of fresh air are the most important factors in inducing disease in the homes. of the poorer classes in our large cities.

Position of the House.-If possible, the house, especially when located on open ground, should face the south or west, in order to secure the greatest amount of sunlight in that portion of the house most constantly occupied. The windows require protection with blinds and awnings in summer, to exclude the heat and glaring

effect of the sun; but in winter the full and free action of sunlight should be secured, at least during a part of each day, because of its purifying influence upon the air of the house.

Foundation and Walls.-The foundations and walls should be as dry as possible, and in damp soils this can be secured only by draining the subsoil below the foundations, and by cementing the foundation walls and cellar floor. If there is no cellar, the floors should be raised about 0.5 meter above the ground, so as to secure thorough ventilation beneath the floor. Dryness of the walls

FIG. 57.-Double foundation walls.

is best secured by means of hollow walls (Fig. 57), or by coating the walls with cement or slate. Bricks are quite porous and absorb a great deal of moisture, and thus render the walls damp. Stone also is porous and retains moisture for a long time, consequently stone houses are liable to be damp.

The Roof. The roof of the house must be carefully constructed and frequently examined in order to prevent leaking. The material composing the roof is of no vital importance so long as it excludes rain. The rain-water falling upon the roof should be conducted away from the

house, so as to prevent the soil of the locality from becoming unduly damp from this cause.

The Interior Arrangement. The interior of the house should be arranged so as to afford the greatest facility for the use of every part of it for the purposes for which it is intended. The stairways should be wide and the steps broad, with easy slope, so as to prevent fatigue as much as possible in going from one floor to another. The rooms on each floor should communicate with each other, or with a common hallway, so as to favor easy access and insure more efficient ventilation.

Size of the Rooms.-The size of the rooms is a matter of the greatest importance in maintaining the purity of the contained air. The detrimental influence of insufficient air space is well known. The point of greatest hygienic importance is not how many rooms a person has, but how much room. A small room may be overcrowded with a single person in it, while a large room is not overcrowded with four to six persons in it. Aside from the matter of cubic space, the question of the dimensions of the room is of the greatest importance. The minimum amount of cubic space allowable in sleeping-rooms per adult person is 25 cubic meters. It is evident that a room 3 meters high is far more easily ventilated than one that is 10 meters high with the same amount of air space. Sleeping-rooms should be at least 2.75 meters high, though a height much in excess of 3 meters is not desirable. A room less than 2 meters in height is not suitable for a sleeping-room. The floor space of a sleeping-room should be at least 8 square meters. The livingrooms of a house should possess a cubic space of about 30 cubic meters.

If the arrangements for ventilation are efficient, the air will require to be changed about three times an hour in a room of 25 to 30 cubic meters capacity, while in a room of only 10 to 12 cubic meters the air must be changed seven to eight times an hour in order to maintain its purity. The living-rooms of a house should be