The principles upon which these arts depend are so mutually involved that it is necessary to consider them together. Much information upon the general subject will be found under the heads Atmospheee, Fuel, Heat, Lungs, Oxygen, and Respiration. While the human body requires to be kept at a temperature of 98° in order to sustain the life processes, it loses heat in a colder medium, like any other kind of matter. This heat is economized by clothing, and by the supply of warmth from external sources. Apartments lose their heat at a rate proportional to the excess of their temperature above the outward atmosphere and the imperfection of the barriers to its escape. Much heat is lost through the thin glass windows. Three fourths of the heat which escapes through the glass would be saved by double windows, whether of two sashes or of double panes only half an inch apart in the same sash. Heat is also lost through walls, floors, and ceilings, at a rate proportional to the conducting power of the materials of which they are composed. Much heat is conveyed away by the currents necessary to maintain combustion; much by the leakage of warm air through various fissures and openings; and where ventilation is attended to, there is further loss by the outflowing currents of vitiated air.

To renew this constantly disappearing heat is the object of contrivances for warming. - The necessity of ventilation results from the vital importance of having pure air to breathe. But as impure air does not affect the senses so directly as a falling temperature, more precaution is needful to guard against it. Pure air contains on an average about 21 per cent, of oxygen, the vital element of respiration, and about one volume in 2,500 or '4 per 1,000 of carbonic acid, a narcotic poison. The air is vitiated in breathing by a double process, the withdrawal of oxygen and the exhalation of carbonic acid. Various causes conspire to deteriorate the air in close inhabited apartments. A person robs of all its oxygen nearly 4 cub. ft. of air per hour, and diminishes its natural quantity 5 per cent, in 80 cub. ft. per hour. The quantity of carbonic acid in the expired breath is 100 times greater than in the natural atmosphere. A person by breathing adds 1 per cent, of carbonic acid to 55½- cub. ft. in an hour, or would vitiate to this extent nearly 1 cub. ft. a minute. Open combustion in a room contaminates the air in the same way. A pound of mineral coal requires 120 cub. ft. of air to burn it, although if the combustion is properly conducted the contaminated air is steadily withdrawn.

But in illumination the products of combustion are accumulated within the room. A candle (six to the pound) will consume one third of the oxygen from 10 cub. ft. of air per hour; while an oil lamp, with large burner, will change in the same way 70 cub. ft. per hour. A cubic foot of coal gas consumes from 2 to 2½ cub. ft. of oxygen, and produces from 1 to 2 cub. ft. of carbonic acid. Thus every cubic foot of gas burned imparts to the atmosphere 1 cub. ft. of carbonic acid, and charges 100 cub. ft. of it with 1 per cent, of this noxious gas. Besides these sources of impurity, subtile streams of effete organic matter are constantly exhaling into the air from the lungs and skin of every living animal. The current from the ventilator of a crowded room has an insufferably nauseous odor, and if passed through pure water quickly renders it putrescent. Thus, morbid, organic poisons, so subtile and minute as to elude chemical detection, may be engendered in the confined air of over-crowded rooms, and become the germs of fever and pestilence. A frequent cause of deterioration of the air in close apartments is the withdrawal of its moisture by heating. While the other ingredients of the atmosphere are constant, its moisture depends upon temperature.

At zero a cubic foot of air will hold •18 grain of watery vapor; at 32° it will contain 2-35 grs.; at 50°, 4.24; at 100°, 19.12; and as the temperature goes still higher, the capacity for moisture rapidly increases. When air is saturated at a given temperature, it will receive no more moisture unless the heat be increased; while if its temperature falls, a portion of its water is precipitated. (See Dew.) In the open atmosphere, where the air is in contact with the moist earth, evaporation and precipitation take place with the rising and falling temperature, but usually within normal or healthful limits. But if air is heated without the requisite addition of moisture, its constitution is disturbed, and it becomes injurious. Air saturated with moisture at the freezing point, and then heated in a room to 100°, has but one eighth its necessary quantity of moisture, and the deficiency represents its drying or parching influence upon the lungs and skin. From tainted air follows tainted blood. Oxygen, the consumer of effete matter and purifier of the system, is withheld, and, the carbonic acid already in the air offering a barrier to its exhalation from the lungs, the vital current is encumbered with the noxious products of bodily waste.

Under these circumstances it is supposed the blood may become a ready prepared soil for the seeds of infection. Atmospheric malaria may be powerless upon a perfectly healthy system, while it would find ready lodgment in a constitution which bad air, by lowering the tone and depressing the vital powers, had predisposed to epidemic disease. Because it requires a given quantity of carbonic acid in the air to produce immediately injurious effects, it does not follow that a much lower proportion does not seriously impair the constitutional energies, and especially the power of resisting disease. Many a case of disease proves fatal on account of an unperceived depression of the sufferer's strength by continued exposure to an atmosphere impure from bodily exhalations. That vitiated air produces intellectual stupor, depression of the feelings, headache, and predisposition to take cold, is proved by very slight observation; and upon few things is enlightened medical experience more unanimous than that it either causes or greatly aggravates the most malignant diseases, such as fevers, inflammations, infantine maladies, cholera, scrofula, and consumption.

The first question in practical ventilation relates to the amount of fresh air that requires to be supplied to occupied rooms; and as the air of such rooms cannot be maintained in absolute purity, the problem is, what is the limit of impurity that may be held consistent with the maintenance of health? Authorities are agreed that the amount of carbonic acid in air tainted by respiration may be accepted as a fair index to the proportion of other and accompanying impurities, so that the question is, how much carbonic acid may be tolerated in respirable air? Parkes, Pettenkofer, Angus Smith, and De Chaumont, agree that the permissible amount of carbonic acid in respired air should not be more than double its normal amount, or 8 per 1,000 volumes. Dr. Parkes maintains that perfect ventilation should keep the proportion down to 6 per 1,000; but that, while it is impossible to prove that a proportion of even 8 per 1,000 volumes produces immediately injurious effects, yet when it rises as high as 1 per 1,000, that is, one tenth of one per cent., the accumulative influence is palpably injurious.

If the maximum impurity allowed is 8 per 1,000 volumes, the exhalation of carbonic acid by breathing would require a supply of 1,500 cub. ft. of pure air per hour, or at the rate of 25 cub. ft. per minute for each person. If the carbonic acid generated by illumination is taken into account, the supply of fresh air would of course require to be greater. The rapid exchange of air involves the difficulty of currents, and this depends much upon the cubical space of the apartment. It is important to avoid perceptible draughts; and if the room be small and the ventilation thorough, the currents are liable to be injurious. Thus with our standard supply of 1,500 cub. ft. an hour, if the apartment has a space of 250 cub. ft., its air must be renewed six times in an hour, while if there is a space of 750 cub. ft., it will require renewal but twice in an hour. Prof. Pettenkofer of Munich has shown that the air in a chamber of 424 cub. ft. (8 ft. high, 8 ft. long, and 6½ ft. wide) can be renewed once in 10 minutes without creating any appreciable air currents. Dr. Parkes maintains that, with natural ventilation, in the English climate, the air can be safely changed only three or four times an hour, which necessitates a larger initial air space.

But the chief practical difficulty in the rapid change of air in small rooms arises from the position of the inlets, which have to be so near the person that draughts can scarcely be avoided. Moreover, in rooms of small space with a large supply of fresh air, it is impossible to obtain a uniform diffusion, as direct currents arise between inlets and outlets. Drs. Parkes and De Ohaumont maintain that the space for each healthy adult ought to be at least 1,000 cub. ft., while it is often not half or a quarter that amount. In estimating the quantity of air required for effectual ventilation, it is to be remembered that the air immediately around the person is becoming constantly and rapidly vitiated by exhalations from the skin and the lungs, and the movement of diffusion requires to be so constant and active as to carry it away and keep the body surrounded by fresh air. The supply may become a question of expense as well as of health, when lower standards must be accepted. - Those exchanges of air which are spontaneously effected in houses by means of various facilities, but without the artificial application of power, are known as natural ventilation. All inequalities of temperature in the air set it in motion.

If the air in a house is warmer than that without, it will escape through windows, doors, cracks, and flues, and the colder outside air will rush in by all chance apertures to maintain the equilibrium. The movements of the external air greatly assist these exchanges. The wind exerts an aspirating action through chimneys and air shafts, creating a partial vacuum in them, which produces up currents. Cowls placed on the tops of chimneys and air flues favor the aspirating power of the wind. Opposite windows afford cross ventilation when opened, if there is much external atmospheric movement. Various outlets and inlets are arranged in the tops of windows, or substituted for the glass panes, or by inserting perforated bricks in the walls near the ceiling, or by valves in chimneys, or passages that can be opened and closed over the doors. Such arrangements are irregular in their effects, and require vigilant attention to make them serviceable. - In artificial arrangements for securing pure air, the agent of warmth becomes the motor of ventilation. The ordinary open fireplace was one of the earliest means adopted to secure both objects. The heat in this case is - entirely radiant, being thrown off directly from the burning fuel or reflected from the sides and back of the fireplace.

It strikes upon the walls, ceiling, floor, and furniture of the room, which are warmed and gradually impart their heat to the contiguous air, thus producing gentle and equalizing currents. As the fireplace is at the side of the apartment, and as radiant heat decreases rapidly in intensity, the warming is very unequal. Near the fire it is hot, and at a distance cold, while a person can be warmed only on one side at a time. The open fireplace is the most wasteful of all arrangements for warming, as a copious stream of air passes up the chimney which takes no part in combustion, but carries off with it much heat. This loss is six sevenths, seven eighths, or even more of the heat produced, so that scarcely 12 or 14 per cent, of the heat is utilized. The coal grate is more economical for warming than the larger wood fireplace, chiefly because it lessens the current of air which enters the flue. Like the fireplace, it is closed on three sides, and these should be of some slow-conducting substance and not of iron, which carries away the heat so fast as to deaden combustion.

The art of burning fuel to the best advantage in open grates is to maintain the whole mass in a state of bright incandescence by preventing all unnecessary abstraction of heat, either by contact of surrounding metal or currents of cold air flowing over the fire. To be burned with economy, that is, to get from fuel the greatest amount of heat possible, it must consume rapidly and with vivid combustion. To insure the greatest heating effect, the air which comes in contact with the fuel should part with the whole of its oxygen. Every particle of air passing through the fire which does not aid combustion obstructs it, first by directly carrying off a portion of the heat, and secondly by cooling the ignited surface so that it attracts the oxygen with less energy. Air entering below a fire rapidly loses its oxygen and becomes contaminated with carbonic acid, both changes unfitting it for carrying on the process actively in the upper region of the fire. If therefore the mass of burning material be too deep, the upper portions burn with least advantage; or if the pieces of coal be large, scarcely any depth of fuel will be sufficient to decompose the whole of the air which arises through the wide spaces. The modifications of fireplace and grate are innumerable.

Some have circular fronts to favor radiation, which is liable to expose the fire to so much air as seriously to obstruct the combustion. An iron plate for a fire-back has been employed to warm an adjacent room behind a fireplace, and for the same purpose grates have been hung upon pivots so as to revolve and thus warm two rooms alternately. In one plan the coal is introduced below the fire, working its way from above downward and consuming the smoke. Grates are often set so low that the radiations pass along parallel with the floor, which is not warmed as it would be if the fire were higher and the radiations struck downward. But, though defective and wasteful for heating, the open fireplace secures considerable ventilation. The magnitude of the open space above the fire, though a source of wasted heat, represents the ventilating capacity of the chimney. Gen. Morin says that a common chimney removes in an hour, on an average, an amount of air which equals five times the capacity of the room it is intended to warm, which is "sufficient in rooms of the usual size to secure a ventilation of over 1,000 ft. of cubic air an hour for each person, supposing there be more than one for every 10 sq. ft. of floor room." Yet it is from the air below the level of the mantel, the purest in the apartment, that the fire is supplied, the vitiated air above being only withdrawn as it cools and descends.

There is also very imperfect diffusion, as the air that is drawn in under the doors and through minute openings streams along the floor to the fireplace, chilling the feet in its way. The changes which of late have been effected in the construction of the fireplace to save heat, the contracting of its dimensions and the lowering of the chimney piece, have been unfavorable to ventilation. The double fireplace is an admirable arrangement both for heating and ventilation. A fireplace of soapstone or other material is set up within another, leaving a vacant space between them into which cold air is admitted from without, warmed, and thrown into the room through an opening or register above. Fireplaces upon this principle, with hollow backs for warming air to be admitted into the apartment, and with hearths and jambs of iron, were constructed by Cardinal Polignac as early as 1713. The latest improvement of this kind is the fireplace or stove devised by Oapt. Douglas Galton of the English army, for use in the barracks. The grates or open stoves, of different sizes, are set in chimney openings to suit the capacities of the rooms. They give the advantage of an open fire with very complete combustion, and the greatest amount of radiant and reflected heat.

The smoke flue is an iron tube set in the chimney and surrounded by air space. On the back of the stove broad iron flanges are cast, so as to present the largest possible heating surface, and these project backward into the air chamber. If the fireplace is built in an external wall, there is an inlet for fresh air from without immediately behind it; but if in an inner wall, a channel of perforated bricks or gratings is laid and passes beneath the flooring or behind the skirting. The fresh air from without, heated in this chamber, enters the room by a louvred opening near the ceiling, or by two such openings, one on either side of the chimney breast. The same principle has been applied to kitchen ranges and stoves in halls; and a cheap cottage grate upon this principle has been devised by Penfold, consisting of well burnt fire clay instead of iron. - Dr. Franklin described the early Holland stoves as plain iron boxes with a flue or pipe proceeding from the top, and a small iron door opening into the room. He also mentions the old German stove as an iron box with one side open, which was set outside of the room, the stove itself projecting through the partition. Smoke and fuel in the apartment were thus avoided, but at the expense of ventilation.

The Franklin stove, invented in 1745, was a great step in advance of the older forms, and has been thus described: "It was a rectangular box of cast-iron plates, open in front except near the top, with a sliding shutter by which the whole might be closed entirely or in part, either for safety or for increasing the draught. The hearth projected in front, and was cast with double ledges to receive the edges of the upright plates, and also with a number of holes, viz.: one in the front part with a regulating valve for admitting air to the fire from an air flue beneath when the shutter was down; one behind the first upright plate in the back, for discharging the air brought under the hearth from without into a narrow rectangular air box that was as long as the width of the stove, and as high excepting the space for the smoke flue over its top; and lastly three holes near the extreme back edge for the smoke, after it had passed over the air box and descended behind it, to enter the flue leading into the base of the chimney.

The air box at its sides was furnished with holes through which the heated air was admitted into the room, and a succession of shelves one above another was provided in this box, reaching not quite across, by which the circulation of the air was extended and it was longer exposed to the heated surfaces before passing out into the room. The back plate of the stove, heated by the descending smoke flue, imparted heat to the air between it and the chimney, the stove standing a little out from the wall. A register of sheet iron was introduced in the descending flue, which could be closed wholly or in part, and check the fire to any desired extent. Thus this stove embodied the principles of the modern air-tight stoves, and the directions Dr. Franklin gave for using it are just as applicable to these." Stoves in the United States are of great diversity of forms, of cast iron, sheet iron, and sometimes of soapstone; while iron stoves, especially for burning coal, are commonly lined with fire brick, which not only increases their durability, but prevents the metal from being overheated. They heat by radiation in all directions from their surfaces; they also heat the air which, rising into the upper part of the room, is diffused by circulation.

Where a room is light, with no loss of heat by outflowing air, and the smoke escapes into the chimney at the temperature of the room, the stove becomes the perfection of economy in heating. Air-tight stoves admit the air in small regulated quantities, so as to produce a slow combustion; but this smothered burning is not eco- nomical. The desirable points in stoves are self-acting contrivances to regulate the draught, accurate fitting of the parts, enclosure of the fire space with slow conductors, and the bringing of all the heated products of combustion in contact with the largest possible absorbing and radiating metallic surface, so that the iron will give out its warmth at a low temperature. But the ventilation afforded by stoves, unless especially provided for by connected warm-air chambers, is very imperfect; only the small amount of air which is necessary for combustion is removed from the apartment, while they unquestionably exert a more or less deleterious action upon the remaining air when made very hot. Precisely what the effect of red-hot iron is upon air or persons is not yet determined. The statement that it burns the oxygen out of the air is erroneous, as this effect is quite trivial.

A stove weighing 84 lbs. and kept exposed to the air at a red heat for 300 days would, if completely burned up, consume the oxygen of but G cub. ft. of air per day, and it would require 19 such red-hot stoves to burn the air as fast as one pair of human lungs. But the ordinary air is contaminated by a great variety of organic matters, carbon particles, filaments of cotton and wool, starch grains, vegetable spores, pollen, volatile emanations, germs of vibriones, bacteria?, and monads, and floating particles of decayed tissues, such as epithelium and pus cells. As Prof. Tyndall has shown, when an electric beam passes through the air it is seen to be loaded with impalpable dirt. Inhabited apartments are charged with this organic dust, and when it comes into contact with hot iron it is decomposed, giving rise probably to the peculiar odor of "burnt air." - The old English cockle stove, introduced by Mr. Strutt toward the close of the last century, warmed houses by the distribution of heated air, and was the progenitor of our hot-air furnaces. It consisted of a cylindrical fire chamber with a dome-shaped head, and was placed in a bed of masonry with a grating and ash pit below.

This part, which from its shape was called the cockle, was enclosed at a little distance by a concentric wall of brickwork, the interval forming a hot-air space. Air introduced from without was thrown into this space against the surface of the iron chamber, and, being heated and rarefied, ascended through openings and was conveyed to the rooms required to be warmed. The modern hot-air furnace consists of an iron stove, which may be variously shaped, and which is surrounded either by an iron or a brickwork case, with a hot-air chamber between. It is situated either in the basement, cellar, or subcellar, while air brought from without (or too commonly from the subterranean apartments) is introduced through proper openings, heated, and conveyed by flues to the different apartments through registers at the base or ceiling. It is urged in behalf of hot-air furnaces that they are out of the way and save space; that they are cleanly, and give but little trouble in attendance; that they are economical in first cost and in consumption of fuel; that they warm the whole house or such parts of it as may at any time be desired; and finally, that they afford at any time an abundant supply of warm fresh air for ventilation.

On the other hand, it is objected that "furnace heat" is in a high degree unwholesome, the hot parching air being unfit for respiration; that deleterious gases escape through the openings of the joints, and even pass through the red-hot metal and are thrown into the stream of air to be breathed; that sparks of fire are thus often carried through the building with the greatest danger of conflagration. But it must be admitted that the evils of hot-air furnaces are chiefly those of faulty construction and of gross mismanagement, as they have been employed for years in many dwellings with entire satisfaction. Much alarm has been created by the French experiments of Deville, Troost, and Morin (1863-'9), proving that carbonic oxide permeates cast iron when heated to redness. It has been also shown that red-hot wrought iron is penetrable in the same way. But although carbonic oxide is a far more poisonous compound than carbonic acid, the amount of it that can permeate a red-hot plate of iron half an inch thick is so very minute that it is not to be brought into comparison with other causes of air contamination. The difficulty with cast-iron furnaces is the liability to flaws in the metal, by which obscure passages are left for the escape of gases, and the great danger of leakage at the joints.

This is obviated by using wrought iron for construction, and where cast iron is employed it should be tested for flaws, and the smallest number of pieces possible should be used. The attempt to do a large amount of heating with small, cheap, lightly constructed furnaces, put up by inexperienced men, which leads to overheating of surfaces and derangement of the structure, is a chief cause of the bad working of hot-air furnaces. They ought to be large and thoroughly made, and placed as near the centre of the house and as low as practicable. The lower the furnace, the greater the possible inclination of the distributing air tubes, and the greater the ascensive force of the air currents. The passages for the entrance of fresh air should be ample, and every precaution should be taken that there are no causes of impurity in the neighborhood of the source of its supply. It has been pointed out how the capacity of air for moisture increases with its temperature. To prevent the parching influence of furnaceheated air, it is necessary to supply the requisite moisture by evaporation in the hot-air chamber. Provisions for this purpose are usually very inadequate. A copper vessel of from 2 to 4 sq. ft. of open water surface should be provided.

Ruttan's air warmer seems to combine the better qualities of stoves and furnaces. It consists of one stove enclosed within another, with sufficient space between to admit a large amount of air, which is brought from without, enters below the floor, passes between the two stoves, and is thrown into the room above. Instead of heating a small quantity of air to a high temperature, the principle of this arrangement is to warm moderately a very large amount of it, and depend upon its rapid exchange to keep the apartments at a proper temperature. It is said to secure the cheapness and simplicity of the stove with the ventilating efficiency of more expensive apparatus. - Water has been used as a vehicle for heat, and its employment depends upon two principles. First, when unequally warmed, its equilibrium is disturbed, and it is thrown into movement. If a tube passes into the upper part of a boiler, and, making a circuit, reenters the lower part, heating the water in the boiler gives rise to a circulation through the tube. The hot water flows away above, and, cooling, descends and returns to the boiler below.

Second, the capacity of water for heat is so great, that is, it holds so large an amount of it, that it gives out a large quantity as it cools, and is thus an admirable medium for its distribution. When the heat of a cubic foot of water is imparted to air, whatever be the number of degrees through which the water falls, it will raise through the same number of degrees 2,850 cub. ft. of air. There are two modes of warming by hot water. In one the circulation takes place through a system of small tubes distributed through the house, and constructed to fit any form and succession of rooms and passages. The pipes are coiled into heaps in various situations, and impart their heat by direct radiation. This is Perkins's arrangement. It has no boiler, its place being supplied by a portion of the pipe coiled up in the furnace, and is a high-pressure method, the temperature of the water rising to 300° or 350°. The warmth diffused from a coil of pipes in a room is mild and pleasant, but no ventilation is provided for. In the other form of hot-water apparatus, the pipes do not ascend to any considerable height above the boiler; there is but slight pressure, and the heat does not rise above the boiling point.

The boiler and masses of pipes are placed in the cellar or basement, and air from without, warmed by passing among the coils of tubing, is distributed to the apartments through flues and registers. As the boiler and tubes contain considerable water, its temperature rises slowly when fire is first applied, and, the quantity of contained heat being large, it cools with equal slowness. Hence the arrangement is well suited to those cases where permanent and unvarying heat is required, as in greenhouses, graperies, etc. Hot-water pipes thus arranged are a source of steady and equable heat. - Steam has long been employed and is increasingly used for heating purposes. It contains a large amount of latent heat, and can be conveyed with facility through pipes to distant points, where, condensing into water, it gives out its heat, and either flows back to the boiler or falls into reservoirs at various points. Irregularities in the fire affect much more sensibly the circulation of steam than that of hot water, and want of attention may lead to condensation, so that when the fire is renewed the steam rushes into the partial vacuum, and meeting the condensed water drives it violently forward with disagreeable noises, and often with a production of leaks.

Steam is applied for heating in two ways, either by coils of pipes or combined metallic sheets (radiators) set up in the various apartments, which warm by direct radiation or by systems of pipes over which air is made to pass, and being heated is sent through the building as in the case of furnaces. Steam radiators give a pleasant heat, but are wholly objectionable from lack of the slightest provision for ventilation. It has been estimated that a boiler adapted to an engine of one-horse power is sufficient for heating 50,000 cub. ft. of space; and "that if steam from the boiler of a working engine is to be used for warming, the boiler requires to be enlarged at the rate of 1 cub. ft. for every 2,000 cub. ft. of space heated to the temperature of 70° or 80°. The amount of heat lost through windows and walls, and by escaping air, has been variously estimated. Dr. Arnott says that in a winter's day, with the external temperature at 10° below freezing, it requires, to maintain an apartment at 60°, a steam pipe heated to 200°, or about one foot square for every 6 ft. of single glass windows; as much for every 120 ft. of wall, roof, or ceiling, and as much for every C cub. ft. of hot air escaping each minute in the way of ventilation.

Hence, a room 16 ft. square by 12 ft. high, with two windows, each 7 by 3 ft., with ventilation at the rate of 16 cub. ft. per minute, would require 20 sq. ft. of radiating surface. Steam for heating is used at a very low pressure, and with suitable precautions is quite safe. For heating large establishments this method has come into extensive use, but, like hot water, it is too expensive for ordinary private dwellings. - Ventilation on a large scale is produced by fans driven by steam power. The fan consists of several vanes, or bladesx inserted into a shaft and made to revolve with it. By the rotation of the blades, the air is driven by centrifugal influence to the circumference, tending to create a vacuum at the centre. If two sides be added to the vanes, having an opening round the axis, when the fan is thrown into revolution, air will rush in through the openings and out at the circumference continuously. If tubes connect these central openings with an apartment, its air will be exhausted; and if the circumference be suitably connected with a room, the air will be driven into it. The same machine therefore becomes an exhaust fan or a blowing fan according to the mode of its use.

Air impelled by a fan maybe heated by various expedients for use in cold weather, but this mode of ventilation is independent of warming, and is chiefly valuable in summer in large establishments, as asylums and hospitals, where many persons are gathered. Ventilating chimneys are flues, sometimes made very high and large, in which fires create powerful draughts that are employed to exhaust apartments of vitiated air. An extra ventilating flue may be constructed adjoining the chimney, warmed by it, and opening into the top of the room, and this may have connecting tubes extending to remote apartments for the ventilation of the whole house. But double outlets to the same apartment rarely work satisfactorily, as the chimney is apt to convert the extra flue into a feeder of the fire, while the smoke escaping from the chimney may be drawn down the flue into the room. The efficiency of ventiducts is augmented by surmounting them with ejectors, which increase their exhaustive action when the wind blows. But under ordinary circumstances, or in the absence of other arrangements, the chimney may be used for conveying away foul air, the velocity of the ascending current giving it considerable exhaustive power.

If therefore an opening is made in the chimney breast near the ceiling, the foul gases accumulated in the upper part of the room rush in, and are carried upward with the current. Yet if from any cause the draught of the chimney be interrupted, smoke is driven back into the room; an ordinary register, requiring personal attendance, being of little use. To remedy this inconvenience, Dr. Arnott constructed a self-acting suspension valve, which is placed in the aperture, and so mounted that a current of air passing into the chimney opens it, while an opposite current closes it. A simple modification of this valve consists of a square piece of wire gauze set in the opening with a curtain of oiled silk suspended behind. - Gas jets may be made important auxiliaries to ventilation. Inserted in the bottom of air shafts, they establish active currents which withdraw the vitiated air, and may be made especially useful on occasions when apartments are unusually crowded. It has been proved by experiment that 1 cub. ft. of illuminating gas can be utilized so as to cause the discharge of 1,000 cub. ft. of air; and as a common gas burner will consume nearly 3 ft. of gas an hour, it would extract from an apartment 3,000 cub. ft. of contaminated air during that period.

By suitable contrivances also the gas lights, which are usually such active causes of deterioration, may not only become self-ventilating and carry off their own impurities, but also aid materially in keeping pure the air of inhabited apartments. Inventors have made successful contrivances for ventilating the burners of chandeliers, but they have hitherto not received the attention they merit. - The point of entrance of fresh air into dwellings is a matter of importance too much neglected. If there be local sources of impurity in the vicinity, or dust, or organic contaminations near the ground, the apertures of ingress should be so placed as to avoid them. It may be well to bring the air from the top of the house. Openings are sometimes made under the eaves leading to channels constructed in the walls which open into the rooms, or furnish air for the warming apparatus. The practical question in ventilation, at what points the fresh air should be introduced into an apartment and the foul air removed from it, is still a matter of controversy. But the points to be secured in regard to openings are, to place them so as to produce the most perfect diffusion of fresh air without sensible draughts, and to have the places of egress as far away from the inlets as possible.

Obviously, if there are large openings or registers of escape at the top of the room, and capacious inlets at the bottom, a strong current from the lower to the higher aperture would be established with imperfect diffusion. The best distribution is effected where the inlets and outlets are numerous, giving rise to many and moderate currents. The general requirements of artificial ventilation are, that the heating arrangements adopted in dwellings, shall be made subservient to the supply of pure air; that definite and ample provision shall be made for the withdrawal of irrespirable air; that equal provision shall be made for bringing in the pure air from without; and that the renewal of the breathing medium by this exchange shall be in relation to the capacity of the apartment, while the details of the arrangements are conformed to the varying circumstances of dwellings, apartments, and occupancy. In its application to assembly rooms, legislative chambers, churches, hospitals, theatres, etc, the subject of warming and ventilation presents complicated and still unsettled problems of science and practice, which form a regular branch of technological study. - See "AManual of Practical Hygiene," by Edward A. Parkes, M. D. (4th ed., London, 1873); "A Handbook of Hygiene and Sanitary Science," by George Wilson, M. D. (2d ed., London, 1873); and Gen. Arthur Morin's treatise "On Warming and Ventilation of Occupied Buildings," translated from the French in the Smithsonian reports for 1873-'4.