VENTILATION is the process of changing the air of rooms and other closed places so that a certain standard of purity may be preserved notwithstanding the vitiation which the air undergoes from the breath of inhabitants, the products of combustion of illuminating agents, and other causes. In estimating the amount of air to be supplied, account must be taken of the standard of purity which is aimed at and of the rate at which vitiation occurs.
Of the various impurities that are found in the air of Carboni inhabited rooms carbonic acid forms the most ready index acid in of the ventilation. The open air of London and otheralr-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 Standard ventilation. It is generally admitted that the air in which of people dwell and sleep should not under any circumstances purity. ^e a[iowe(j t0 contain more than ten parts in 10,000. This has been accepted as the permissible proportion by Car-nelley, Haldane, and Anderson, after an extensive examin-ation of the air of middle and lower class dwellings.1 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,2 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.
Bate of consumption of
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.3 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 Commissioners4 require that the supply be not less than 1200 cubic feet per man per hour. Pettenkofer recommends 2100 cubic feet, and Morin5 con-siders that the following allowances are not too high :—
Gas lights add to the vitiation of the air of rooms at a Vitiation rate which may be roughly estimated by treating onebybum-cubic foot of gas burnt per hour as nearly equivalent to gas' 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.
Hospitals
Middle class houses
Barracks
Good secondary schools
London board schools
Workhouse dormitories
London lodging-houses
One-roomed houses6

The purity of the air of a room depends, of course, to Cubic some extent, on the proportion of its cubic capacity to capacity the number of inmates. The influence of capacity is, of rooms-however, often overrated. Even when the allowance of space is very liberal, if no fresh air be supplied, the atmo-sphere 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 inade-quate ; but a large room has at least the advantage over a small one that it offers more chances for the casual en-trance 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:—
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, Ventila-but chiefly by actual currents of air. The experiments oftion by Pettenkofer have shown that air passes to a very sensible d'^ion
¡111(1 by
6 Mean of 29 measurements by Carnelley, Haldane, and Anderson, loc. cit.
extent through the substance of brick walls. In houses currents built of stone the movement of air through the walls must of air. 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 com-munication 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

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 unfre-quently 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 Motional resistance to its flow : it has also to set the air entering the room in motion and to overcome frictional resist-ance 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 con-ditions the air enters in small streams or narrow sheets, ill-distri-buted 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 com-bustion, 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 chirr ney 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 ceil-ing with soot near the opening. If a valve near the ceiling be pro-vided, 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 Inlets, 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 th
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 Tobin
of which is due to Mr Tobin of Leeds, the fresh air enters verti- tube,
cally upwards. The usual arrangement of Tobiu'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 enter-ing 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 dis-

charge outside the products of combustion); or the tube may draw its supply, not directly from the outer atmosphere, but from a hot-air 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 and door, provision is made for them in the walls, the advantage
of a current entering vertically may still be in some FIG. 3.—Short degree secured by help of certain makeshift eontriv- Tomn *ul)e-anees. 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;

FIG. 4.—Ventilating inlet fitted to window.
FIG. 5.—Ventilating inlet at foot of door. The arrows indicate the direction of the current of air when the door is closed
ne, 6th ed., p. 171.
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 dis-tance of half an inch or so from the surface of the door; the air then enters in a vertical stream. Arrange- As an example of the systematic ventilation of dwelling-rooms ments in on a large scale, the following particulars may be quoted of arrange-barracks; ments 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 to-gether 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 lO.OOO.1 In public In the natural ventilation of churches, halls, and other large buildings, rooms we often find air admitted by gratings in the floor or near it, —an offensive plan, since it fouls the air, besides causing objection-able 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
1 De Chaumont, in Farkes's
The most complete safeguard is to place in the
A. 4. x
= L^r\ /
f
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 ventilation is to guard them against down-draught through
the action of the wind. Numberless forms of cowl have been de-
vised with this object, and often with the further intention of
turning the wind to useful account by making it assist the up-current
of foul air. Some of these exhaust cowls are of the revolving class : Exhaust
a hood or trumpet-shaped mouth, opening horizontally and sup- cowls,
ported about a vertical axis so as to be free to turn, is kept facing
away from the wind by means of a large vane. To make the wind
help the up-current, a horizontal conical tube is fixed within the
cowl, pointing towards the wind and discharging through the
trumpet-mouth of the cowl, where it exhausts by suction, on the
principle of the jet-pump. Revolving cowls are liable to fail by
sticking, and, apart from that, when the wind blows in shifting
gusts the}' cannot respond quickly enough to its changes of direc-
tion to prevent it from occasionally
blowing down. Fixed cowls are to
be preferred ; they are designed in
many forms, of which Mr Buchan's
may be cited as a good example.
Fig. 6 shows this ventilator in
horizontal section: aa is the ver-
tical exhaust flue through which
the foul air rises; near the top this
expands into a polygonal chamber,
bbbb, with vertical sides, consisting
partly of perforated sheet-metal
plates ; outside of these are fixed
vertical curved guide-plates, c,c,c,c;
the wind, blowing between these
and the polygonal chamber, sucks
air from the centre through the per-
forated sides. Perhaps no form of cowl is entirely free from liability
to down-draughts,
exhaust flue a set
of flap-valves open-
ing only outwards.
Fig. 7 shows the ar-
rangement of exit-
valves employed by
Mr Buchan ; the
valves a, a, a are
flaps of oiled silk,
working on a wood-
en "rid bb which is ^IG' Ex't valves in exhaust flue,
inclined enough to let them hang free of it when no current is
passing; beyond them is a door closing-valve c, worked by hand by
the cord d ; and the whole is enclosed in a box, with glass sides ee,
through which the action of the valves may be seen. When the
outlets are guarded by valves of this type they may discharge
through a plain box with louvred sides ; an exhaust cowl, however,
may still be used with advantage to assist the ventilation under
favourable conditions of the wind.2
The two things that supply motive force in automatic ventila-tion—the difference of temperature between inner and outer air and the wind—are so variable that even the best arrangements of inlets and outlets give a somewhat uncertain result. To secure a strictly uniform delivery of air, unaffected by changes of season or of weather, the influence of these irregular motive forces must be as far as possible minimized, and recourse must be had to an artificial method of driving the air.
Artificial or Mechanical Ventilation.—This finds application on Artificial the largest scale in the ventilation of collieries, by methods which ventila-are fully described under COAL (vol. vi. pp. 70-71) and MINING (vol. tion. xvi. p. 460). Motive force is supplied to the up-cast shaft either by a furnace at the base, which heats the rising column of air, or (in more modern practice) by a centrifugal fan, such as Guibal's, exhausting air from the top. The long galleries and workings through which the air has to be driven oppose so much resistance that the pressure required to move a sufficient volume of air is im-mensely greater than is ever necessary or desirable in the artificial ventilation of buildings.
see S. S.
A broad distinction may be drawn between what are sometimes Vacuum called vacuum and plenum methods of artificial ventilation. In and the former, as in colliery ventilation, the motive force is applied at plenum the outlets : air is drawn from the rooms, and the pressure of their methods., atmosphere is less than the pressure outside. In the latter the motive force is applied at the inlets : air is pushed in, and the pres-sure within the room is greater than outside. The plenum method has distinct advantages : it makes the air escape instead of coming


in as a cold draught at every crevice and casual opening to the outer air ; it avoids drawing foul and mouldy air from sewers and basement; and with it, more easily than with the other, one may guard against the disturbing influence of wind. In the plenum method the air is driven by pumps or by fans ; in the vacuum method pumps are rarely if ever used : suction is produced by fans or by heating the column of air in a long vertical shaft through which the discharge takes place. Water jets and steam jets have also been employed to impel the air. Extrac- Extraction by a hot-air shaft is a common mode of ventilating tion by hospitals and other public buildings. Heat is applied by a furnace hot-air or stove at the bottom of the shaft, or by coils of hot-water,,or shaft. steam pipes, which should not extend up the shaft farther than can be helped. In the lecture theatre of the Paris art conservatory, ventilated by Morin, where this means of extraction is employed, fresh air enters through the ceiling and foul air is drawn oil' through the floor from under the seats ; this reversal of the natural direction of the current is of course only possible when a sufficient external motive force is applied. The House of Commons furnishes another example: there the air, niter being warmed and moistened, or cooled by water spray, as the state of the atmosphere may require, is admitted through large gratings in the floor, which are covered by porous matting to prevent draughts ; outlets from the top of the House lead by Hues to the Victoria tower, where a furnace maintains the current in an up-cast shaft. In theatres and other buildings lighted by clusters of gas jets or sun-lights at the ceiling the lights may be turned to account as effective ventilating agents by letting the foul air escape through shafts placed over them, which they heat at the base. What is known in America as the Ruttan or Smead system of ventilation, successfully applied in many schools there, employs a hot-air shaft to furnish motive power. In warm weather a stove at the base of the shaft is used to heat the column ; in cold weather the exhaust air from the rooms is so much warmer than the atmosphere outside that the up-cast shaft acts without additional heating. This is in.fact an example in which the classification of systems into natural and artificial breaks down. The supply of fresh air is warmed as it enters by passing through chambers containing tubular metal stoves ; the outlets are at or near the floor level. A curious feature in the arrangements is that the foul air, in passing to the up-cast shaft, is drawn through the privies, where it desiccates all discharges. Extrac- Extraction by fans presents no features requiring special remark, tion by A favourite fan for the purpose is the Blackmail propeller, the fans. nearly flat form of which allows it to be readily placed in walls and partitions. One of these fans, 4 feet in diameter, when driven at a speed of about 330 revolutions per minute, is said to discharge 15,000 cubic feet of air per minute with an expenditure of one-horse-power. Though this is a good performance, it should be observed that for ventilating purposes, where air has to be driven in large volume with low velocity and under low pressure, fans, while they have the advantage of being less bulky, are less efficient than pumps, for they require that the air in passing through them should move much faster than in other parts of its course, and much of the energy of this motion is wasted in eddies. When fans are used to blow air into buildings, they should deliver into a chamber of considerable size, that the air may become nearly still before it passes into the distributing flues. Loss of power may be avoided to some extent by receiving the air in a channel which gradually enlarges as it leaves the fan. Fans in The plenum method, with fans to drive the air, is exemplified plenum on a large scale in the ventilation of St George's Hall, Liverpool, ventila- where there are four large fans in the basement, driven by a 10-tion. horse-power steam-engine. The building is heated by passing the air through chambers containing coils of hot-water and steam pipes ; after the air is warmed it is moistened by injecting steam, and provision is made for washing it by water-spray before it reaches the fans.
When fans are used, either with suction or with pressure, the amount of the current is not strictly independent of those variable motive forces which are the sole agents in natural ventilation ; the case is analogous to that of an electric circuit in which several sources of electromotive force are at work, assisting or opposing one another. The fan may be the main agent in circulating the air ; but differences of temperature, and at times the action of the wind, may make large variations in the resultant effect. The case is different when pumps are used. A certain quantity of air is delivered at each stroke, and the only effect of these irregular forces is to make the power required to drive the pump sometimes greater and sometimes less. Provided there are no casual inlets and outlets, the amount of air supplied is known with certainty ; the ventila-tion under these conditions is sometimes described as positive. Positive Good recent examples of positive plenum ventilation are to be plenum found in Dundee University College and in a number of schools ventila-in Dundee and Aberdeen, where the arrangements have been de- tion. signed by Mr W. Cunningham. Some of these are ordinary double-acting reciprocating pumps, driven generally by water engines. The pumps are rectangular wooden boxes, stiffened by iron ribs, and provided at top and bottom with inlet valves, consisting of a number of short waterproof cloth flaps working against a vertical wooden grid. The piston, which is also of wood, has a vertical travel; it is held in place and worked by wire ropes above and below, which lead over pulleys to the water motor ; and the piston is balanced by a counterweight on the descending branch of the upper rope. A piston 5 feet square, with a stroke of 5 feet, works at 20 strokes per minute and delivers 150,000 cubic feet per hour.
In other instances, where the volume of air is greater than could easily be dealt with by common pumps, Mr Cunningham uses re-volving pumps of the Root's blower type (shown in transverse vertical
8). At the discharg-easily by ry havina
section through the revolvin Dundee College a battery of fl-ing over 150,000 cubic feet a gas engine of two-hojse-powi coils of Perkins's high pressure hot - water pipes in the main distributing flues. The inlets are flat upright tubes extending up the side walls to a height of nearly 6 feet, and open at the top. Ample proof of the advantage that results from giving a vertical direction to the entering current is supplied by the success of Mr Cun-ningham's arrange-ments, where this form of inlet is ex-clusivelyadopted. Al-ternative outlets are generally provided in the end walls, one group near the ceiling, another a few-feet from the foot. They are fitted with doors which allow one
other to be closed ; the high-level outlets are used in warm weather, when the fresh air that comes in is com-paratively cool; the low-level ones are used in cold weather, when the fresh air, having been heated before it enters, would tend to rise and pass out too directly if the outlets near the ceiling were open. The outlet shafts communicate with a louvred tower or turrets on the roof. Each room receives a volume of air equal to its cubic capacity in about 12 minutes, so that the atmosphere is completely changed five times in an hour. The inlets are propor-tioned to do this without allowing the velocity with which air enters to exceed 6 feet per second.
The "iEolus " water-spray ventilator of Kind and Mestern is an Water-example of a mechanical ventilator using a jet of water to impel the spray air. A nozzle at the top of a circular air-shaft delivers a conical venti-sheet of water, which impinges on the sides of the shaft a littie way lator. below and carries down with it a considerable stream of air. This ventilator is used either to force air into rooms or to draw it out; in the former case a small gas-stove is often added to heat the supply.
For the ventilation of greenhouses and hot-houses, see vol. xi. p. 231.
The advantage of ample and systematic ventilation is Organic not to be measured only by the low proportion of carbonic matter acid it secures. Carbonic acid is not the only test of "I* vitiation; it is not even the most dangerous impurity. 0r»ani"smt Another criterion of the foulness of close air is the amount in air. of oxidizable organic matter it contains; still another,—and a most valuable one,—is the number of micro-organisms, especially of bacteria. The micro-organisms may be de-termined by Hesse's method of slowly passing £>, given volume of the air to be examined through a tube coated inside with beef jelly; the germs are deposited on the nutrient jelly and each becomes in a few days the centre of a very visible colony. In outside air the number of micro-organisms, as tested in this way, varies greatly : it

is often less than 1 per litre (61 cubic inches); in -well-, ventilated rooms it ranges from 1 to 20; in close school-rooms as many as 600 per litre have been found. The elaborate researches of Carnelley, Haldane, and Anderson on the air of dwellings and schools illustrate well the value of this test. One of the uses to which they have put it has been to compare schools known to be well ventilated (by mechanical means) with schools ventilated at hap-hazard or not ventilated at all. A large number of trials were made in each case; in the mechanically ventilated schoolrooms the average number of micro-organisms was 17 per litre, and in the others 152. Results of great in-terest were obtained by the experiment of stopping the me-chanical ventilators for a few hours or days. Tested by the proportion of carbonic acid, the air of course became very bad; tested by the number of micro-organisms, it remained comparatively pure, the number being, in fact, scarcely greater than when ventilation was going on, and far less than the average in " naturally ventilated " schools. This proves in a striking way the advantage of systematic ventilation. The bad effect of a foul stagnant atmosphere is cumulative. An habitually close room acts as a nursery of micro-organisms, which a casual flushing with fresh air will not remove; an habitually well-ventilated room is kept in great measure clear of these dangerous inmates, and its atmosphere may be occasionally overtaxed without causing the number of them to be seriously increased, (J. A. E.)


Footnotes

1 Phil. Trans., 1887, vol. clxxviii. B, p. 61. In school-rooms well Tentilated by mechanical means these authors found 13 parts of car-bonic acid in 10,000 of air, which they consider a limit permissible in rooms of that class, though not in dwelling-rooms.
2 Proc. Roy. Sue, 1875, vol. xxiii. p. 187.
3 This estimate is based on the observations of Pettenkofer, Angus Smith, and Parkes.
4 Report, 1861.
6 See Études sur la Ventilation, Paris, 1863 ; also Proc. Inst. Mech. Eng., 1867, p. 63.

6 Mean of 29 measurements by Carnelley, Haldane, and Anderson, loc. cit.

The absence of proper inlets for air in a house where several fires are burning involves a danger that is much more serious than other effects of bad ventilation. When the air which is required to take the place of that discharged by the chimneys can only struggle in through small openings, the pressure within the house falls considerably below that of the outer air, the water traps under basins and closets are liable to be forced, and foul air is drawn in from every leak in soil-pipe or drain. The writer has found a house drawing what seemed to be its main supply of " fresh " air from the public sewer, through a defective joint between the soil-pipe and the (untrapped) house-drain.
Report of the Bairacks Commissioners, 1861.
See observations by De Chaumont, in Parkes's Hygiene, 6th ed.,
p. 173.

4 When the air is not filtered, and when it has been warmed before entering, the vertical direction of the stream is readily traced by dust which is deposited on the wall in a nearly upright column, spreading slightly fan-wise as it rises. With cold air the deposit of dust is com-paratively slight. The difference is due to the fact noticed and ex-plained by Mr John Aitken, that air quickly deposits any suspended particles when it is brought into contact with a surface colder than itself, but retains them in suspension if the surface be warmer than the air {Trans. Roy. Soc. Edin., vol. xxxiii., 1884, p. 239). Another domestic illustration of the same fact is given by the greater dustiness of walls and furniture in a stove-heated room than in a room heated by an open fire.

2 For an account of tests of various forms of ventilating cowls Hellyer, The Plumber and Sanitary Rouses, 4tli ed., 18S7.

For examples, see Morin, op. cit., or Proc. Inst. Mech. Eng., 1867, p. 61.
The arrangements are similar to tho^e introduced by Dr Reid in the
temporary Houses of Parliament, and described in his Treatise on Ventilation and Warming; see also Tomlinson's Warming and Ventilation, p. 265. In recent years pumps have been added, through which air may be forced into the building ; but the hot shaft is generally used.
:f See, for example, Morin's account of the ventilation of the Theatre Lyrique. Paris. i Proc. Inst. Mech. Eng., 1863, p. 194.

Phil. Trans.. 1887, volTclxxviii. B, u. 61.