Articles of furniture, walls, floors, etc, in greater quantities than where an adequate movement of air is freely permitted. Naturally there may be various reasons why at different times more or less dust will be deposited in rooms, but, in a properly-ventilated room, the finer particles of dust will not readily accumulate.

The ground on which the house is built is often a fruitful source of contamination to the air within. Not only excessive moisture or damp, but also noxious gases, may be drawn up from the ground by the warmth of the house. Hence the necessity1 of draining the subsoil, of rendering the basement-walls as impervious as possible, of covering the site with an impervious ground-layer, and of laying on all walls, immediately above the ground, a thoroughly damp-proof course.

A basement story under living-rooms is generally an advantage, provided it is kept dry and clean, and is well supplied with fresh-air inlets and up cast Hues. In winter it often happens that, when the house is closed, an adequate amount of air is not supplied to the several apartments, and when tires are lighted, the basement-air is, to a large extent, drawn upon to take the place of that which ascends the Hues. A fusty smell within a house is often traceable to dampness, or to an accumulation of rubbish, in the basement. To remedy such a state of things, the basement should be cleaned, dried, and provided with suitable air inlets and outlets, and at the same time separate air-inlets should be constructed to each room in the house, with the fireplace flues as exits.

Where there is no basement, and a space is left between the ground and the floor immediately above, equal care should be taken to keep the space dry and clear of all contaminating influences, because when windows, doors, and ventilators are closed, such spaces will certainly be drawn upon to supply air to the rooms above. Many people would be surprised to find, if the floor were removed, what an amount of filth there often is in such places, and they might then realize why the rooms are close and stuffy, and why unhealthiness has frequently occurred among the inmates. When such air-spaces are properly constructed and cleared of all rubbish, it is still necessary (in order to prevent the decay of wood and the stuffiness alluded to) to provide for change of air therein, and the height of perfection would be to do so quite independently of the rooms above, but such is rarely practical. A separate flue or flues carried up with the smoke-flues, and air-inlet gratings suitably placed around the walls, will generally suffice.

Foul gases from drains and waste-pipes are a dangerous kind of air-contamination, and particular attention should be given to the principles laid down in the sections on "Sanitary Plumbing", "Sanitary Fittings", and "Drainage", in order that the house may be free from all these deadly emanations. Even when all waste-pipes discharge over trapped gullies, particular care should be taken that the air-inlets to the rooms are as far from them as possible, as occasionally the gullies may choke and overflow, or the standing water in the traps may evaporate and allow the free escape of air from the drains.

1 As explained in Section II. vol. i pp. 56, 69-71, and 75-85. - ED.

Faulty construction of the house, as explained in Chapter III (Drain-Pipes)., is another source of air-contamination, and in this connection the following paragraph from Section II. (vol. i. page 61) may be quoted: dirt is so prevalent, it behoves the architect to avoid as far as possible all ledges, nooks, and crevices, and all unseen spaces which could give it lodg-ment. Considered in the light of cleanliness, the ordinary floor, with its plastered ceiling below and gaping boards above, is radically wrong; so also is the confined space so often provided between the ground-layer and ground-floor; so also arc lath-and-plaster partitions, hollow walls, and indeed all details of building-mstruction which provide space invisible and inaccessible to the householder. Sooner or later dirt finds its way to these dark places, and vermin breed and wander there, safe from the housemaid's broom and the cat's eager paw."

Free admission of daylight to enclosed spaces intended for occupation appears to be necessary for maintaining them in a condition suitable for healthy residence, and therefore light is an important factor in connection with effective ventilation. Its purifying action upon the atmosphere is most marked, and consequently should be encouraged.

Until the introduction of the incandescent electric light, air-contamination suited from all methods of artificial illumination, these artificial lights being the result of combustion, by which complex impurities are thrown off into the atmosphere, carbonic acid predominating.

Candles, particularly those of common make, will, light for light, contaminate the atmosphere more than most illuminants.

Animal and vegetable oils come next, and then coal-gas. In oil-lamps, very much depends upon the form of lamp and burner, and the currents in the air of the apartment. Irregular and rapid movements in the air cause smokiness and imperfect combustion, which result in contamination of the atmosphere by products which are injurious to health when breathed.

The introduction of better illuminants has tended to a demand for more and more light, and the greater the ease and economy with which it can be used, the larger the quantity consumed. At the beginning of this century, a single candle would be considered sufficient for a room of moderate dimensions - later, an oillamp. Then probably three gas-burners would not be thought excessive, and now that people are generallv becoming accustomed to the electric light, even more gas-jets may be demanded where the electric light is not available. All this continually-iiii-reading demand for more powerful illumination has had a marked effect upon the ventilation of d welling-rooms.

The introduction of coal-gas resulted in the demand for lofty rooms, whirh in turn require the use of more gas for lighting them, and yet in but few cases have additional inlets and outlets for causing increased change of air been provided; consequently, with a larger capacity and no letter means of causing change of air, ventilation cannot be so good as in rooms less lofty, - i.e. of less cubic capacity, - because change of air in the latter can, under similar conditions, be more quickly brought about Various methods for conveying away the products of combustion, when gas is employed for illuminating purposes, have been adopted, but they cannot as a rule be relied on to act in a satisfactory manner under varying conditions of the outer atmosphere, and are frequently affected by the Lighting of a fire, and the owning or closing of a door or window. Sunlight-burners have been extensively used, but the great heat emitted therefrom precludes their use in any but lofty apartments, and in them an excessive consumption of gas is required to secure the requisite illumination. .Moreover, they act as a powerful means by which air is extracted from the apartment, and unless the greatest care be- exercised in providing suitable inlets, they will cause down-draught in fireplace flues; and in cold weather it is difficult to regulate the inflow of air, required to replace that which they are the means of removing, in such a manner that discomfort may not be caused to the occupants of the room. The incandescent gas-burners now largely employed give an excellent light with less contamination of the air than in the case of ordinary burners.

The incandescent electric light has fortunately been so far perfected and cheapened, that it is placed within the reach of many who desire a strong illuminant without contamination of the atmosphere. An additional advantage which it possesses is the comparatively small rise in temprature involved, compared with the illuminating power, so that the ordinary means provided for securing ventilation are not materially affected when the electric light is turned on. It is in fact an ideal illuminant in connection with means for securing efficient ventilation, because it neither contaminates the air nor materially influences its movements. The methods of generating and applying it will be described in a subsequent section.

Wherever change of air in an inclosed space, such as a room, is retarded, and there are contaminating influences, it is not alone the air which is vitiated, but every surface therein becomes more or less tainted, and the longer such contamination takes place, the longer will be the time required (during which more frequent change of air is necessary) before such space and the articles within it will again become pure and the room rendered suitable for healthy occupation. Hence it is that every careful housewife realizes the necessity for the frequent removal of everything which is likely to cause air-contamination in and about her dwelling, as well as for the frequent airing of unoccupied rooms by throwing wide open the windows and doors.

Where there hae been infection or continued contamination, it may be advisable to employ powerful disinfectants, not, as a rule, for making the atmosphere itself wholesome, but more generally for purifying the surfaces, objects, and parts of a building inaccessible to any other cleansing influence. For this purpose volatile substances arc employed, which should be made to penetrate all cracks and crevices, ample time being allowed, so that the thorough destruction of what is injurious may take place. Fluid disinfectants may be employed for all accessible surfaces which would not be injured thereby. Many so-called disinfectants possess a strong odour but little antiseptic power; they are worse than useless in connection with ventilation, because they mask bad odours, which otherwise might indicate the sources of contamination and lead to their removal1