s the art of providing any confined air or apartment with an adequate supply of air, in a position suitable to the purposes for which it may be required. The air is usually introduced in a stream which maintains a freshness and purity of atmosphere in the place thus ventilated, any contaminated air being withdrawn when pure air enters.

Numerous operations of nature and of art, and more especially during combustion, respiration, and the decay that attends the putrefaction of animal and vegetable matter—wholesome products are evolved; and so deleterious are these to animal life, that death is the necessary consequence when they accumulate in a more concentrated form around the person. In a smaller proportion, they produce an endless variety of discomfort or disease, from the most trifling sense of languor or debility to the more violent apoplectic headache, for the time suppressing all attempts at exertion either of mind or body, while on other occasions a slow and insidious action gradually undermines the constitution, and induces a permanent loss of health. In extreme cases death is induced, more or less rapidly, according as the oxygen of the atmosphere is withdrawn, or impregnation of poisonous gases communicated to it.

The air of respiration is the great paludum vitae: we depend upon it no less than twelve hundred times an hour, on average, for nourishment and support, during the whole period of our existence; we consume oxygen, and replace it by carbonic acid; and were it withdrawn from us or changed in its qualities, death would inevitably ensue. The air acts incessantly, not only on the blood as it passes through the lungs, but also on the surface of the body; and disease and death may ensue from an unwholesome atmosphere in contact with the skin, even when the lungs are supplied with pure air. Well attested cases are recorded where severe oppression has attended the action of an amount of impurity so small as from 1-15000th to 1-5000th part of sulphured hydrogen gas; while the absolute amount of impurity in air tainted by miasma is so excessively minute, that its precise nature, as well as the minuteness of its weight in the most pestilential atmosphere, is unknown.

A supply of a fresh and wholesome atmosphere may accordingly be ranked among the first and most essential necessaries of life. In the atmospheric ocean which rests on the surface of the terraqueous globe, a perpetual movement, or natural ventilation, is sustained on a great scale by numerous causes, more especially however by the unequal action of the rays of the sun on the equator and at the poles; the colder air moving along the surface from the poles to the equator, while the warmer air from the equator ascends and proceeds in a contrary direction towards the poles. These great and primary currents are modified in endless variety by the attraction of the sun and moon, the rotation of the earth upon its axis, the relative effect of the land and water, the ever-varying influence of local temperature, volcanic action, meteoric phenomena, more especially the evaporation and deposition of moisture, the electrical condition of the air and of the surface of the earth, and the innumerable changes that attend chemical action in the mineral kingdom, as well as in those that occur in the organic world. The animal and vegetable kingdoms not only contribute to the movement of the air, but are at once the great causes of the most important changes induced upon it, and of its restoration to its former composition; the animal kingdom consuming its oxygen and producing carbonic acid, while in the vegetable kingdom, the great tendency is to absorb carbonic acid and replace oxygen. But where these great movements are interrupted by local causes, or an undue accumulation of vegetable and animal debris takes place, there the due balance is not sustained, pestilential effluvia contaminate the air, and were it not for the wind, the rain, and the impetuous storms which from time to time visit such localities, and the operation of a peculiar diffusive power, in consequence of which no gas can accumulate permanently on the surface of the earth, whatever may be its specific gravity, they would in general become at last as fatal as the valley of death in Java, or the carbonic acid springs in Bavaria, in the grotto del Cano, and other places, which are so notorious for their destructive atmosphere.

In selecting a site either for a house, a city, or any establishment where persons are crowded together, too much attention cannot be paid to its natural ventilation; an aspect towards the south, a dry gravelly soil and moderate elevation, securing efficient drainage and freedom of access to the air, with protection from offensive currents, and an immunity from local impurities, are the great desiderata.

In the open air the temperature is generally under that of the body; and the air expired from the lungs, and also that in contact with the surface of the body, being expanded by the heat which it receives, escapes from its levity, and a fresh stream is immediately supplied; but when the body is confined within a limited space, special means must be employed to sustain a constant renewal of fresh air.

This artificial ventilation, it is obvious, must be modified and adapted to the peculiar circumstances of each individual case, and the state of the external air which is supplied. It is a subject therefore of great extent, and as various in its details, as the climate in which man lives, the habitations in which he dwells, the occupations in Ventilation which he is engaged, the food upon which he lives, the means of protection against heat and cold which he can command, and the peculiarity of constitution which he may present. We shall therefore content ourselves, in the remaining space allotted to this article, by pointing out the leading facts connected with the more prominent objects which it includes in reference to the frame of man. It must not however be forgotten that this branch of science is as yet in its infancy, that the history of numerous points connected with the motion of aerial fluid, is only beginning to be minutely investigated; and that, in a practical point of view, it is as yet so imperfectly attended to, that it is impossible to turn to any city without seeing discomfort, disease, and even death, from time to time induced by ignorance of the laws of ventilation; and in assemblies of every variety, the whole audience is not only too frequently subjected to extreme uneasiness, but the tone of the mental faculties, and the capacity for exertion and attention are often affected by the state of the atmosphere.

I. Source of Air.—This cannot be too particularly examined. Many of the public buildings in this country are supplied with air from the lowest levels, principally with the view of facilitating the movement of the air through the flues. Hence at night, in clear weather especially, when the cold produced by radiation from the ground is great, and more or less at all times, cold air is supplied from the surface of the ground, or the public streets, loaded with the offensive emanations which they evolve, and which become much more disagreeable when subjected to the action of heat in a warming apparatus. By taking air from the highest attainable point, avoiding merely the immediate vicinity of chimneys, and any ingress of air at the same level, a much purer atmosphere is secured; and it may be conducted downwards to any required depth with a flue. In extensive public buildings situated in localities noted for the inferiority of their atmosphere, the erection of air towers to draw down a comparatively wholesome atmosphere from a height of two or three hundred feet, would be an object of great importance. In large towns, such as London and Manchester, where local impurities abound, and particularly when fog and frost are observed at the same time, special means may be adopted for removing those that are most offensive. Filter beds may be constructed for air on the same principle as for water; but few of the more simple arrangements of this kind that have been sustained for any length of time, have been found more desirable than filtration through any porous texture to exclude suspended blacks (soot), and in some cases also washing with water (particularly lime water), to condense sulphurous acid and other gases.

II. Amount of Air necessary for Ventilation.—Few subjects present a greater diversity of practice than this. From two to four cubic feet per minute for each person have usually been considered a large supply, but this is far beyond the amount usually allowed. A very slight examination will however show that even this amount is too small; for if the process of respiration be accurately examined, it will be observed that a cubic foot of air or more is involved or mixed and contaminated with the air discharged from the lungs at each expiration, independently of that affected by the skin. Such a supply, therefore, is at least desirable, were the air always at a mild and genial temperature. According to Dr D. B. Reid's experiments, where the effects of variable quantities of air were tried upon numbers included in an experimental apartment, not less than ten cubic feet of air per minute should always be allowed when it is at an agreeable temperature; and to sustain the atmosphere in all its freshness and purity, even a much larger quantity is at times desirable. In the present Houses of Parliament, from 36,000 to 50,000 cubic feet per minute have occasionally been given in warm weather to one apartment alone, or about 60 feet per minute to each individual in a crowded house. This question, however, of the amount of air necessary for ventilation, taking the deterioration produced by the human frame alone into account, is exceedingly complicated, and is influenced by a vast variety of circumstances, of which the following are the most important. In the preceding and following remarks, a temperature of 65° may be considered as the average most generally desired when there is a steady but gentle movement in the air. It is not so much the amount of supply with which life can be sustained, which is here investigated, as that which it may be desirable to afford, so as to maintain the system in its highest vigour and perfection.

1. The purity of the air supplied.—In general, the less pure the air, the greater the amount necessary for ventilation, especially if it be loaded with moisture, and charged with offensive exhalations from the lungs and skin. But cases do occur, when an atmosphere is so largely charged with poisonous effluvia, that it becomes an object to use as little as possible to avoid this source of contamination, if it be not removed by special arrangements.

2. The temperature.—This is much the most important circumstance affecting the supply necessary for ventilation. When the air is very cold, and the moisture of the breath is condensed in hoar-frost as fast as it escapes from the lungs, a proportion of air, extremely small compared with the usual desirable allowance, will be sufficient for ventilation. Air, under ordinary circumstances, is below the temperature of the body in most parts of the globe. It therefore acts as a cooling power. But the higher its temperature, and the more nearly it approaches that of the body, the larger is the quantity required to produce an equivalent cooling power. Further, in warm weather the air is charged with more moisture than in the cold season, while the body is at the same time more exhalent of moisture. These and other circumstances tend to render the supply of air desirable in warm weather, far greater than the mere arithmetical increase in the temperature would at first sight appear to indicate. But here we must advert to the popular error that the temperature, as indicated by the thermometer, is a proper guide to the quality of air, in respect to the warmth that may be most agreeable. The temperature, as a moment's reflection will show, is no indication, unless the velocity of movement and chemical qualities of the air, especially in reference to moisture, be also taken into consideration. A small quantity of air stagnant, and at 32°, may not cool the body more than a larger supply at 40°, 50°, 60°, 80°, or 90°, and, indeed, at any temperature below that of the living system. A large quantity of air not so cold may be made to produce the same amount of cooling effect as a less quantity of the colder air. It is equally obvious that a large quantity of air charged with much moisture, but not saturated, may produce the same amount of evaporation from the system as a less quantity of air containing little moisture associated with it.

3. Idiosyncrasy.—A very wide range of experiments has shown that the constitutional peculiarities of different individuals vary as much in respect to the amount of air desired, and the temperature at which it is preferred, as in respect to food or drink. In the houses of parliament, no temperature below 62°, or above 76°, has been demanded for several years. It is often very difficult to determine, however, how much is due to absolute peculiarity of constitution, and what is dependent on the circumstances of the moment, more especially, the state of occupation or excitement, the time that has elapsed since any refreshment was taken, and the nature and quality of the repast, the clothing in use, the previous exposure, the temperature, moisture, and other circumstances affecting the quality of the air. The brilliancy of the illumination also affects the supply of air required by some constitutions. The force of habit is nowhere exemplified in a more marked manner than in the amount of contamination which different individuals can bear. Some miners pass usually much of their time in air in which a candle does not burn, though a lamp may be maintained in combustion in the same air.

**Moisture in the air.**—Air, in winter, usually requires the addition of moisture when introduced into any apartment and warmed, as at this season it has deposited a large portion of the moisture associated with it in summer. Were it warmed by the approach of summer, it would, in this country, gain moisture from the surface of the earth or the ocean, and thus acquire more of the pleasing and agreeable qualities which a summer atmosphere presents; but, if warmed and introduced into any apartment without the previous addition of moisture, then, having had its power of action upon moisture increased without receiving a corresponding supply, it absorbs moisture with extreme rapidity from the surface of the body and the lungs, and, taking away an undue proportion, produces a harsh and disagreeable impression. The injection of steam into the air, or the evaporation of water from shallow metallic pans, placed over the heating apparatus, removes the defect.

On the other hand, moisture is frequently communicated to the air with the view of cooling it, by the reduction of temperature attending evaporation.

In some climates, again, a redundancy of moisture forms one of the greatest sources of oppression to the European constitution, and is regarded as one of the most powerful causes in developing the activity of miasmatic influence.

The influence of different degrees of moisture in the air is various upon different constitutions as that of different temperatures. Further, some individuals exhale moisture almost solely by the lungs, while, in others, the skin is equally active. According to the relative conditions of the living system, the air, in reference to moisture, may either exhale, or absorb moisture from it.

I. **Equalization.**—Air entering by doors and windows, and rushing in a well-defined stream, so as to produce local currents, may be exceedingly offensive; while, if divided into innumerable streams, by causing it to pass through a porous texture, having the most extensive possible surface, it may escape so insensibly as not to be perceived. Air ought always to be admitted in this manner, on such a distance from those upon whom it is to act, that its movement may be greatly moderated before it reaches the person. Wherever a proper supply of air is admitted, this equalization of movement is essential, more particularly in crowded apartments; and the greater degree to which it is carried, the larger is the quantity of air which the system can bear. Nothing is more common than to see apartments ventilated effectually, so far as may be necessary for removing the foul air; but this very ventilation induces a most offensive series of chilling draughts, if means be withheld for warming the larger supply now rendered necessary, and for introducing an effective equalization. In rooms for invalids, this subject becomes of great consequence, especially in diseases of the chest. And, generally speaking, to a great number of constitutions, unequal currents are more dangerous and offensive than an oppressive atmosphere.

II. **Nature of Heating Power.**—Few circumstances require more care and attention in connection with ventilation than the nature of the heating power employed. The open fire, so much prized in this country from its lively and cheerful appearance, exerts also an agreeable effect on the animal system, by the light radiated along with the heat which it evolves, and the movement of air which it creates, as it draws off the air and ventilates the apartment in which it is placed; but, in point of economy of fuel, or facility of regulation, so as to maintain an equal temperature, it is perhaps the least desirable of all kinds of heating apparatus. The great beauty, therefore, which its appearance presents, the absolute purity of the heat which it conveys by radiation, and the extreme facility of access which it affords, so important for a variety of different purposes, ought to be contrasted with the attendance which it requires, the dust and ashes which it leaves, and the tendency, when neglected, to produce back-smoke, if the circulation be not maintained in proper force. In connection with ventilation, the following points require special attention in the construction of the common fireplace. 1. It should be provided with an independent supply of air entering in its immediate vicinity, to be employed when heat is required in any apartment without changing the air there, as in warming the apartment before it is occupied, or moderating offensive currents near the fireplace. 2. An open fireplace, unless the air enters from the ceiling, often produces little or no ventilation above the level of the chimney-piece, and, even then, it does not afford the best and purest atmosphere. 3. The air above may be comparatively stagnant, and offensive in the extreme, from lamps and the products of respiration, while a fresh current moves along the floor to the fire-place.

Of other arrangements for heating apartments, the mild hot-water apparatus, when the water is always under a boiling temperature, affords the most perfect and equal diffusion, when properly arranged; a point of great importance in producing equality of ventilation. Stoves of low combustion, presenting an extensive surface at a moderate temperature, varying from 100° to 200°, according to their size, as the Russian, Prussian, and Swedish porcelain stoves, or Dr Arnott's stove, come next in order. Those varieties should be preferred which are provided with chimneys, both for the ingress and egress of air, having no communication with the apartment in which they are placed, otherwise, when worked most economically, they are all liable, occasionally, to evolve carbonic acid, if they are not adjusted to the smoke chimney by persons who thoroughly understand their operation. Stoves and other apparatus, where the iron is heated to a high temperature, may be in many cases more economical than the preceding; but from the manner in which they affect the air, they are not so conducive to health, and greatly impede all regularity of ventilation, the hot air from them ascending rapidly to the ceiling, while a cold atmosphere, almost unaltered, is often left below.

In public assemblies, subject to great and sudden fluctuation of attendance, hot and cold air chambers are provided, from which any supply of warm or cold air may be obtained; and a mixing chamber, for mingling the various proportions that may be supplied, must also be constructed.

V. **Source of Movement.**—An alteration of specific gravity in the air, dependent on an alteration of temperature, produced by expired air and the warmth of the body, is the greatest, the more natural, and the most economical source of movement under all ordinary circumstances. When the openings for ingress and egress are arranged on the best footing, any ventilated apartment may be compared to a piece of apparatus in which the current of air, entering rapidly by a narrow channel, expands with the greatest possible diffusion into a slow moving stream, occupying the entire area of the place to be ventilated, and gathering together again in a smaller channel, by which it escapes, where the velocity of movement increases as the area diminishes. In ordinary apartments, where nothing more can be afforded, two openings, at different levels, will always give much relief; the one usually admitting cold, and the other, which should be as high as possible, discharging hot and foul air.

A movement in the air may be induced by mechanical means, more especially by pumps and fans, or the impetus communicated to it by high-pressure steam. All these means may be seen advantageously employed where peculiar facilities are presented, as in manufactories.

The moving power of air itself is constantly taken advantage of, both for forcing and exhausting air. The wind-sail and the cowl are familiar illustrations where this power is resorted to; but the continuity of its action cannot be depended on, and it is perpetually varying in intensity. A large shaft or chimney, with a good fire, is more generally employed at present than mechanical means, though both may occasionally be introduced in the same place with advantage. In ships, Captain Warrington has proposed the introduction of a fan; and a number of experiments was made on board the Benbow a few years ago by Dr Reid, who proposed the use of the same instrument, but who did not consider that it would attain all the purposes desired, unless conjoined with a system of diffusion and equalization, which has been partially introduced in the steam boats Albert, Wilberforce, and Soudan, constructed for the African expedition. There, arrangements have also been proposed for purifying the air, as the expedition traverses the most unwholesome districts.

VI. INGRESS AND EGRESS OF AIR.—Doors and windows, the only sources for the ingress of air in ordinary apartments, should be avoided as a means of introducing air in all new buildings, and independent apertures and flues provided, entirely under the control of valves, by whose action the amount of ingress can be regulated with precision. Further, in every apartment foul air should be led from the greatest altitude by similar independent flues. The ordinary construction often interferes seriously with ventilation, foul air descending sometimes in one and escaping in another. An alteration in the structure of houses, introducing one principal chimney, in which the flues from all the apartments ultimately join, would be a great improvement in this respect; and even for whole streets and districts one large shaft would be sufficient. To any who studies attentively the movement of currents of air, numerous other changes will present themselves as being equally desirable in the habitations of the rich and of the poor. In public buildings, the foul air, from hundreds of apartments, will be removed by a single shaft.

The reader who is anxious to extend his knowledge of this subject, will find much information in the Parliamentary Reports, particularly on the Ventilating and Acoustic arrangements of the present House of Commons, on the Ventilation of Mines, in many of the Reports on Education and on Manufactures, in which the health of the children has more particularly engaged attention; in the Reports on Improving the Health of Large Towns, and in some Statistical Reports on the Health of the Army and Navy. Tredgold on Warming and Ventilating, is a work of great value, and the different memoirs and treatises of D'Arcet, present numerous important details. In Dr Arnott's work on Warming and Ventilating, a full description of his stove is given; and in Hood's, and also Richardson's treatise on the same subjects, the different varieties of hot water apparatus are explained. In Dr Reid's Chemistry of the Atmosphere, and Illustrations of Warming and Ventilating, the result of numerous experiments is given on the ventilation of public buildings and private dwelling-houses, and the ventilation of ships, manufactories, &c.

The laws of the communication of heat, the electrical condition of the atmosphere, the diffusion of gases, the fluctuations of the barometer, the indications of the hygrometer; the plenum movement, when the pressure of the air within, from mechanical means, exceeds that without the apartment ventilated; the vacuum movement, when the reverse is induced; the downward and lateral current, especially the former, which is so important in protecting works of art exposed in public assemblies; the precautions necessary where it may be introduced, the influence of different varieties of clothing, of medicated atmospheres, of moist and dry air baths at different temperatures, and of fumigation, are all objects of inquiry that bear practically on ventilation. The peculiar arrangements in various mines, hospitals, and manufactories, especially where offensive vapours are condensed or destroyed, likewise afford many valuable illustrations of its practice.