Of the various impurities that are found in the air of inhabited rooms carbonic acid forms the most ready index of the ventilation. The open air of London and other large inland towns contains about four parts by volume of this gas in 10,000 of air. In the country and in towns near the sea two or three and a half parts in 10,000 is a more usual proportion. Authorities on ventilation usually take four parts in 10,000 as the standard for pure air, and use the excess over that quantity in estimating the adequacy of the air supply. But they differ as to the proportion to which the carbonic acid may be allowed to rise in good ventilation. It is generally admitted that the air in which people dwell and sleep should not under any circumstances be allowed to contain more than ten parts in 10,000. This has been accepted as the permissible proportion by Carnelley, Haldane, and Anderson, after an extensive examination of the air of middle and lower class dwellings. De Chaumont, judging by the rough and unsatisfactory test afforded by the sense of smell, concluded that the air of a room ceased to be good when it contained eight volumes of carbonic acid in 10,000 of air, and recommends that six parts in 10,000 be taken as the maximum permissible in good ventilation. Parkes, in his Manual of Hygiene, quotes observations which point to an equally exacting standard as desirable.

The rate at which an adult respires carbonic acid varies widely with his condition of repose, being least in sleep, greater in waking rest, and very much greater in violent exercise. As a basis on which to calculate the air necessary for proper ventilation we may take the production of carbonic acid by an adult as 0∙6 cubic feet per hour. Hence he will produce per hour, in 6000 cubic feet of air, a pollution amounting to one part of carbonic acid in 10,000 of air. If the excess of carbonic acid were to be kept down to this figure (1 in 10,000), it would be necessary to supply 6000 cubic feet of fresh air per hour; if the permissible excess be two parts in 10,000, half this supply of fresh air will suffice; and so on. We therefore have the following relation between (1) the quantity of air supplied per person per hour, (2) the excess of carbonic acid which results, and (3) the total quantity of carbonic acid present, on the assumption that the fresh air that is admitted contains four parts by volume in 10,000 :— [table]

Thus, to preserve the lowest standard of purity tolerated by sanitarians, ventilation must go on at the rate per person of 1000 cubic feet per hour, and 3000 cubic feet per hour are required to preserve the higher standard on which some authorities insist. Parkes advises a supply of 2000 cubic feet of air per hour for persons in health and 3000 or 4000 cubic feet for sick persons. The English Barracks Improvement Commissioners require that the supply be not less than 1200 cubic feet per man per hour. Pettenkofer recommends 2100 cubic feet, and Morin considers that the following allowances are not too high :— [table]

Gas lights add to the vitiation of the air of rooms at a rate which may be roughly estimated by treating one cubic foot of gas burnt per hour as nearly equivalent to one adult person, so far as the production of carbonic acid is concerned. Thus an ordinary burner, giving a light of about twenty candles and burning four cubic feet of gas per hour, uses the air of three or four men.

The purity of the air of a room depends, of course, to some extent, on the proportion of its cubic capacity to the number of inmates. The influence of capacity is, however, often overrated. Even when the allowance of space is very liberal, if no fresh air be supplied, the atmosphere of a room quickly falls below the standard of purity specified above; on the other hand, the space per inmate may be almost indefinitely reduced if sufficient means are provided for systematic ventilation. Large rooms are good, chiefly because of their action as reservoirs of air in those cases (too common in practice) where no sufficient provision is made for continuous ventilation, and where the air is changed mainly by intermittent ventilation, such as occurs when doors or windows are opened. It must be borne in mind, too, that no room is hermetically sealed. In the absence of proper inlets and outlets casual ventilation goes on through every chink and cranny, and even by diffusion through the plaster of the walls. The ventilation given in this way is generally most inadequate; but a large room has at least the advantage over a small one that it offers more chances for the casual entrance of fresh air, as well as a larger wall-surface through which diffusion may occur. It has also the advantage that a greater volume of air may more easily be passed through it than through a small room in a given time, without causing disagreeable draughts. A general idea of the cubic capacity per inmate, allowed by law or by custom in certain cases, is given in the table below :— [table]

To realize the need of provision for ventilation, it is only necessary to compare these figures with those already given for the rate of consumption of air. Taking the lowest permissible degree of purity (10 parts of carbonic acid in 10,000), we see that, if no fresh air were allowed to enter, the dweller in a middle-class house would make the atmosphere of his room unfit for breathing after occupying it for 1 hour, and that the sleeping rooms of the poor would fall below the standard in 13 minutes.

The atmosphere of rooms is changed partly by diffusion, but chiefly by actual currents of air. The experiments of Pettenkofer have shown that air passes to a very sensible extent through the substance of brick walls. In houses built of stone the movement of air through the walls must be insignificant; but, as regards individual rooms, what is chiefly important in this connexion is the percolation through dry plaster, causing an exchange of atmosphere to occur between the inside of the room and the space within the lining of the wall, which is generally in communication with other parts of the building and with the external air. In order that the atmosphere of a room should be changed by means of air currents, three things are necessary,—(1) an inlet or inlets for the air, (2) an outlet or outlets, and (3) a motive force to produce and maintain the current. One might think it needless to enumerate such obvious requirements were it not that, in providing appliances which are intended to act as ventilators, one or other of the three essentials is not unfrequently overlooked. In systems which are distinguished by the general name of mechanical or artificial ventilation special provision is made for driving the air, by fans, or by furnaces, or by other contrivances to be described more fully below. In what is called natural ventilation no special appliance is used to give motive force, but the forces are made use of which are supplied by (1) the wind, (2) the elevated temperature of the room’s atmosphere, and (3) the draught of fires used for heating. A careful distinction should be drawn between cases in which these motive forces are skilfully taken advantage of, by the use of proper inlets and outlets, to give the best result attainable without special appliances for driving the air, and other cases, unfortunately too common in practice, where the ventilation is left to take care of itself. In a fair comparison of a mechanical with the natural system we should exclude examples in which the ventilation is haphazard, or in which as near an approach to no ventilation is reached as the conditions of modern architecture will permit.

Domestic Ventilation.— The chief agent in domestic ventilation is the chimney; when a bright fire is burning in an open grate, it rarely happens that any other outlet for foul air from a room need be provided. The column of hot air and burnt gases in the chimney is less heavy, because of its high temperature, than an equal column of air outside; the pressure at the base is therefore less than the pressure at the same level outside. This supplies a motive force compelling air to enter at the bottom through the grate and through the opening over the grate, and causing a current to ascend. The motive force which the chimney supplies has not only to do work on the column of air within the chimney, in setting it in motion and in overcoming frictional resistance to its flow: it has also to set the air entering the room in motion and to overcome frictional resistance at the inlets. In many cases the latter part of the chimney’s work is the more considerable of the two. From want of proper inlets air has to be dragged in at a high velocity and against much resistance, under the doors, between the window sashes, and through a hundred other chinks and crevices for which we have to thank imperfeet carpentry and half-seasoned timber. Under these conditions the air enters in small streams or narrow sheets, ill-distributed and moving so fast as to form disagreeable draughts, the pressure in the room is kept so low that an opened door or window lets in a deluge of cold air, and the current up the chimney is much reduced. If the attempt is made to stop draughts by applying sand-bags and listing to the crevices at which air streams in, matters only become worse in other respects; the true remedy of course lies in providing proper inlets. The discharge of air by an ordinary open fire and chimney varies widely, depending on the rate of combustion, the height and section and form of the chimney, and the freedom with which air is entering the room. About 10,000 cubic feet per hour is probably a fair average, about enough to keep the air fresh for half-a-dozen persons. Even when no fire is burning the chimney plays an important part in ventilation: the air within an inhabited room being generally warmer than the air outside, it is only necessary that an up-current should be started in order that the chimney should maintain it, and it will usually be found that a current is, in fact, passing up.

When a room is occupied for any considerable length of time by more than about half-a-dozen persons, the chimney outlet should be supplemented by others, which usually take the form of gratings in the ceiling or cornices in communication with flues leading to the open air. Frequently these openings are protected from down-draught by light flap valves of oiled silk or sheet mica, opening outwards. To increase the efficiency of the ventilating action of the chimney, Dr Arnott advocated (in 1849) that an opening should be made near the ceiling, into the chimney, guarded by a flap valve of this type, with the object of providing a direct exit for the foul warm air that gathers in the upper region of a room, especially when gas is burnt. To make Arnott’s valve of much service it should be larger than the size usually supplied; even then it has the drawback that, notwithstanding the protection given by the valve, enough back-flow may occur to blacken the wall and ceiling with soot near the opening. If a valve near the ceiling be provided, it is probably better in most cases to lead its outlet shaft direct to the open air than to lead it into the chimney.

With regard to inlets, a first care must be to avoid such currents of cold air as will give the disagreeable and dangerous sensation of draught. At ordinary temperatures a current of outer air to which the body is exposed will be felt as a draught if its velocity exceeds 2, or at most 3, feet per second. The current entering a room may, however, be allowed to move with a speed much greater than this without causing discomfort, provided its direction keeps it from striking directly on the persons of the inmates. To secure this, it should enter, not horizontally nor through gratings on the floor, but vertically through openings high enough to carry the entering stream into the upper atmosphere of the room, where it will mix as completely as possible with warm air before its presence can be felt. A favourite form of inlet is Sheringham’s (see fig. 1). When opened it forms a wedge-shaped projection into the room, and admits air in an upward stream through the open top. It is usually placed near the ceiling; but a lower place in the wall would be better. Other inlets are made by using hollow perforated blocks of earthenware, called air-bricks, built into the wall; these are often shaped on the inner side like an inverted louvre-board or Venetian blind, with slots that slope so as to give an upward inclination to the entering stream.

In another and most valuable form of ventilator, the introduction of which is due to Mr Tobin of Leeds, the fresh air enters vertically upwards. The usual arrangement of Tobin’s tube (shown in front elevation and section in fig. 2) is a short vertical shaft of metal plate or wood which leads up the wall from the floor level to a height of 5 or 6 feet. Its lower end communicates with the outer air through an airbrick or built opening in the wall; from its upper end, which is freely open, the current of fresh air rises in a smooth stream, clinging, as it were, to the wall, and scarcely changing its direction until it has passed far above the level of the opening. Various forms of section may be given to the tube: if placed in a corner it will be triangular or segmental; against a flat wall a shallow rectangular form is most usual; a lining of wood forming a dado may even be made to serve as a Tobin tube by setting it out a little way from the wall. The tube is often furnished with a regulating valve; but this is a doubtful advantage, as it tempts the inmates to stop the ventilation for no better reason than that the room is cold; in exceptional circumstances, such as the presence of an invalid, the opening may be stopped or reduced by laying a board over it. Contrivances are occasionally added for cleansing the entering air. A muslin or canvas bag hung in the tube, or a screen stretched diagonally across it, may be used to filter out dust; the same object is served in some degree by forcing the air, as it enters the tube at the bottom, to pass in close contact with the surface of water in a tray, by means of a deflecting plate. These complications have a double drawback: they require frequent attention to keep them in order and by putting resistance in the way of the stream they are apt to reduce the efficiency of the ventilation. The air entering by a Tobin tube may be warmed by a coil of hot pipes within the tube or by a small gas-stove (provided of course with a flue to discharge outside the products of combustion); or the tube may draw its supply, not directly from the outer atmosphere, but from a hotair flue or from a room or corridor where the air has been already warmed. The opening should always be about the level of a man’s head, but the tube need not extend down to the floor: all that is essential is that it should have sufficient length to let the air issue in a smooth vertical current without eddies (fig. 3).

These inlets are at once so simple and effective that no hesitation need be felt in introducing them freely in the rooms of dwelling-houses. When no special provision is made for them in the walls, the advantage of a current entering vertically may still be in some degree secured by help of certain makeshift contrivances. One of these, suggested by Dr Hinkes Bird, is to open one sash of the window a few inches and fill up the opening by a board; air then enters in a zig-zag course through the space between the sashes. Another plan is to have a permanent vertical slot between the sashes by making the top of the lower sash stand out a little from the bottom of the upper one. Still another plan is to have a light frame of wood or metal or glass made to fit in front of the lower sash when the window is opened, forming virtually a Tobin’s tube in front of the window (see fig. 4, where a portion of the frame is broken away to show the position of the window sash). This last contrivance allows the fresh air as ready access as may be wished; and, unlike the others, it is still effective when the blind is drawn down. A Tobin’s tube, however, is better placed against a dead wall than below a window, for the ascending current is liable to be broken by the window recess and by the down-draught which a window causes by its cooling action on the air of the room. The principle of giving entering currents an upward direction is turned to useful account in a simple contrivance (see fig. 5) for preventing the disagreeable cold draught which comes along the floor of a room from the chink beneath the door. The clearance under the door is made a little greater than usual, and a thin piece of wood is set on the inner side as close as possible to the floor and at a distance of half an inch or so from the surface of the door; the air then enters in a vertical stream.

As an example of the systematic ventilation of dwelling-rooms on a large scale, the following particulars may be quoted of arrangements that have been successfully used in English barracks for more than twenty years. One or more outlet-shafts of wood are carried from the highest part of the room, discharging some feet above the roof under a louvre; the number and size of these shafts are such as to give about 12 square inches of sectional area per head, and the chimney gives about 6 square inches more per head. About half the air enters cold through air-bricks or Sheringham valves at a height of about 9 feet from the floor, and the other half is warmed by passing through flues behind the grate. The inlets taken together give an area of about 11 square inches per head. A fairly regular circulation of some 1200 cubic feet per head per hour is found to take place, and the proportion of carbonic acid ranges from 7 to 10 parts in 10,000.

In the natural ventilation of churches, halls, and other large rooms we often find air admitted by gratings in the floor or near it, —an offensive plan, since it fouls the air, besides causing objectionable draughts, unless the temperature is very carefully regulated. The inlets should consist, like Tobin’s tubes, of upright flues rising to a height of about 6 feet above the floor, from which the air proceeds in vertical streams. If the air is to be warmed before it enters, the supply may be drawn from a chamber warmed by hot-water or steam pipes or by a stove, and the temperature of the room may be regulated by allowing part of the air to come from a hot chamber and part from outside, the two currents mixing in the shaft from which the inlets to the room draw their supply. If a basement or story below the room to be ventilated is available, a good plan is to carry the inlet tubes vertically down through it and warm the air in them, so that the height of the warm column assists the flow. Outlets usually consist of gratings or plain openings at or near the ceiling, preferably at a considerable distance from points vertically above the inlet tubes. One of the chief difficulties in natural...