Houses built of excessively porous materials, on the other hand, may be much more unhealthy, because, in rainy weather, the tendency is for them to absorb much wet, which, if it does not actually penetrate to the interior and do definite damage to the walls and decorations, produces a chilling effect within, principally brought about by evaporation, which always results in a lowering of temperature and causes condensation within, as previously de-cribed.

My opinion is that both extremes are wrong, and that, if more attention were paid he selection of suitable materials for the walls of our homes, there would be little call for special ventilating appliances1, - other than windows and fireplace- flues, - particularly in localities where cleanliness reigns within and without, where no overcrowding takes place, and where (are is exercised in opening windows freely at suitable times. On the other hand, it if unreasonable to expect that efficient ventilation can be secured in badly-constructed dwellings, built with unsuitable materials, or where there is neither cleanliness within nor without, or where overcrowding is permitted.

1 One of the chife advantages of "ventilating appliances" is that they can be easily regulated.--ED.

The construction of the Internal partitions both horizontal and vertical, and the condition in which they arc left in those portions generally out of sight, have much to do with movements of air within, which do not necessarily result in efficient ventilation. In fact, it may as a rule be considered that such movements of air from one apartment to anothcr are objectionable, and are often most harmful, if there is air-contamination within any part of the building, which can be communicated to other parts.

Let us now make an imaginary examination of a house of ordinary type, erected (say) a few years ago, before sanitary science had received so much attention as of late. This I suggest because I have learnt by experience the advantage of examining closely the causes of defective ventilation, an intimate knowledge of which will enable one at times to remedy the evil with comparatively slight trouble and at small outlay.

We will select a moderately cold day, and enter an apartment in which a fire is burning briskly. Close all windows, doors, and inlet-ventilators, and hold a lighted taper to the key-hole in the door, to the cracks around door and windows and under the skirting board, and to the cracks in the floor-boarding, and probably the inlet of air will be detected, more or less, at each.

Stop all noticeable crevices and cracks, and a further search will detect others by which air can enter, and even if they can be closed, it will still be found that air must be entering to replace that which is continually ascending the flue; although probably, if the closing of inlets has been thorough, the fire will not be burning so briskly, nor will the velocity of the up-current in the flue be so rapid, but some change of air must still be taking place in the room.

Many people hope to lessen draughts by closing all cracks and crevices, but by doing so they are more likely to create others more cutting, unless suitable provision be made for the admittance of an ample supply of air in a suitable position, so that it may at once be equally diffused throughout the apartment This can readily be illustrated in a room with a fire boning and all inlets from the outer air closed. Slightly open the door, and probably anyone near the opening will experience an unpleasant draught, but fully open the door, and unless there is a direct blow of cold air from the outside, no perceptible draught will be felt, although a larger volume of air may be passing through the room than when the door was only partially open.

Let us continue our examination of the room, supposing it to he on the ground-floor, with no basement under, Inspect the space under the floor-boards.

Is it clean and freely supplied with fresh air? If not, that drawn into the room will be more or leSs contaminated. Air-circulation there must be under wood floors, or dry-rot will set in; care, however, must be exercised so that air-gratings for providing ventilation under ground-floor rooms be placed so as to avoid drawing air from impure sources, and householders should be ever on the watch to prevent the accumulation of filth in the neighbourhood of their houses, particularly near air-inlets. The continuous ventilating anti-damp course1 has much to recommend it as an anti-damp course, and as a means for providing an air current under all floors, but, on account of the modern requirement that all wastes discharge visibly into gully-traps, and the chance of accumulations of filth being permitted around some portion of a building, I am of opinion the better plan is to make the anti-damp course solid, and to select positions for the air-grates where contamination of the air is least likely to take place.

In examining a room on an upper floor, consider first the construction of such floor. Remove some of the floor-boarding, which perhaps is not tongued and grooved, so that each joint is open, and if not very wide, may in part have become stopped up with accumulated filth. The space between the floor and ceiling of the room below is probably open. If meals have been partaken of in the room, crumbs and organic particles resulting from the habitation of human beings, animals, Ac., may be detected among the dirt and dust which will have accumulated Washing the floor will moisten these particles, which will then putrefy and throw off deleterious gases, to be drawn into the apartment through the cracks and crevices in and around the floor. Moreover, the plaster ceiling is porous and often cracked, so that, when the joints of the floor-boarding are also open, there is actual air-communication between the two rooms, and even if direct currents from one to the other are not set up, the property of diffusion of gases will certainly come into play and cause the air of the two rooms to mingle.

The lower rooms of a dwelling are generally employed as sitting-rooms, those above as bedrooms; consider what may, under such conditions, take place during the winter months. Several people may occupy the ground-floor rooms, and a considerable quantity of gas may be employed for lighting purposes. Windows, doors, and ventilators may be closed, and even under moderately favourable conditions of ventilation, it will be found that the air of the top portion of the room will become so highly vitiated that it cannot be breathed for long with comfort. The tendency of this foul air will be to make its way through ceiling and floor to the rooms above, to which, after a time, some of the occupants of the sitting-room will retire to sleep, and will have again to breathe the same air, which may already have passed through their lungs and in other Ways have become vitiated.

1 Fig. 29- and 30. Vol i. p. 85.

Now notice where the joists pass through the inner walls or partitions, as there may be openings through to the adjoining rooms; trace them along, and if there is a water-closet on the them floor, you may find a clear air-way through to it between the joists. If there is any accumulation of tilth near the .Inset-basin, or if any of the joints in connection with the closet and branch soil-pipe leak, the foul air may he drawn along the floors to other rooms, especially when fires are burning in them.

The space usually inclosed under the roof, and not utilized for dwelling pur] becomes an air-chamber, generally connected in a more or less direct manner with the several apartments. Too often such spaces become receptacles for all sorts of lumber, and are frequently allowed to become extremely dirty. In such cases, air drawn into the rooms therefrom can scarcely escape con-tamination.

The construction of the roof has a marked influence upon the temperature within a building. A mere covering of slates afford- little protection except from wet, and in many cases not even from that. Then it is that the walls and ceilings become .saturated with water, and cold and dampness, decay, and con-sequent deterioration of the air of the dwelling follow. In summer slates become heated and communicate heat to the air below, and in winter the reverse takes place, by which the ventilation of the building is constantly being affected in an adverse manner. When the slates are laid with very open joints, even an ordinary wind will blow through and cause discomfort, but in stormy weather the effect may be to rob the whole building of the greater portion of its heat, and to cause draughts which are positively unbearable.

Well-made plain tiles, which, while not being absolutely porous, should not be too much compressed in the process of manufacture. or they will partake more of the properties of slate, afford better protection against changes of temperature without; yet having regard to the efficient ventilation of a dwelling, both slates and tiles should be laid upon a non-conducting material, such as boarding, felt, pugging, or the like, in order to prevent changes of temperature outside having any rapidly-marked effect upon the air within. Care must. however, be taken to secure reasonable change of air within the roof-space, independently of the apartments of the house intended for oceupation.1

1For illustrations of houses specially built for the purpose of enjoying thorough ventilation, see Plates XX. XXI. ami XXII.; some account of these houses will be found in a supplementary note to this section on pp. 224-227. - Ed.