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 exhalant 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. Idiosyncracy.—A very wide range of experiments has shewn 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 report, 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 mixed manner, than in the amount of contamination which different individuals can bear. Some miners pass naturally 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 proportion of the moisture associated with it in summer. When it is 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 a 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, or at 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.
V. 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 maintains, 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 tempera-
ture, 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 especial attention in the construction of the common fire-place. 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 fire-place. 2. An open fire-place, 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 great, 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 fanner; 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.