Popular Science Monthly/Volume 11/June 1877/Relation of the Air to the House We Live in

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613740Popular Science Monthly Volume 11 June 1877 — Relation of the Air to the House We Live in1877Max Joseph von Pettenkofer




WE shall devote this evening to the consideration of some hygienic functions of the house.[2] On the whole, the house has the same hygienic object as our clothing: it has not only to keep up the intercourse with the atmosphere surrounding us, but to regulate it according to our wants. No more than our clothing ought the house to be a contrivance for excluding us from the air outside. In some of their forms we can also trace a certain mutual transition. The cloak and the tent are cousins. The heavy circular cloak of former times might well be styled a portable tent, and the tent a fixed cloak; both have their necessary openings. So the hat may be considered the roof of our clothing, and the roof the head-gear of the house.

We may then naturally suppose that those materials which are advantageous for the building of our habitations must stand in some-what the same relations to air, water, and heat, as the materials we use for our clothing. Walls allow air to pass through them, and they must do so to a certain degree, if we are to preserve our health within them with some comfort, and without injury. Current opinion is certainly opposed to my assertion about the permeability of walls to air, even more so than to that about the permeability of our clothing; but it is easy to show that current opinion labors under an error which has no other basis than the insensibility of our senses to the movement of the air, if the same is less than nineteen inches per second. This is the cause of the fallacy that no motion of the air takes place. Just as well might we deny the earth's rotation round its axis at the rate of more than a quarter of a mile per second, because we are not in the least aware of this tremendous velocity. Only very late and slowly have our minds opened to the conviction that after all the earth moves round the sun, and not the sun round the earth, and that our eyes had all the while been mistaken. There must exist something of a higher nature, of a greater power, than our sensuous perceptions, and that is science, which examines and probes our perceptions. Science has not the least power over Nature; she cannot command any alteration in Nature, cannot give it any laws-she can only recognize the laws of Nature. But science changes the notions of man, and often reverses them. Ideas and notions based on science enrich us, partly directly, partly indirectly, with new means of making use of natural laws. It was not till astronomy had found and determined celestial mechanics that the human mind was enabled to begin that development of the mechanical element which is the pride and the power of our period as compared with former times.

If, then, we are hopefully satisfied with endeavoring to increase our insight, our science of the things that are, the benefit will not fail to come, and everything is beneficial of which man learns to make use. This requires time—often a very long time—as old experience teaches.

The task of science is to lay hold of everything perceptible, and to penetrate it—the small as well as the great. The insect and its life is just as interesting to science as the elephant, and therefore I believe that I may occupy myself with that air which flows through our walls, although its motion is not recognized by our sensations.

We may conclude from many facts that walls are permeable to air. No one maintains that houses have water-tight walls, and everybody knows that masonry is easily penetrated by water. Wherever a wall is in perpetual contact with water, it becomes so soaked that at last water comes out in drops on the other side. Certainly, where water can pass, air must pass much more easily, because air is seven hundred and seventy times more light and movable than water. It is very easy to construct water-tight apparatuses and vessels, but very difficult to make them air-tight. Still people are surprised when they hear of a change of the air through a wall; they see, of course, and feel the water in the wall, but of the air in it their senses have no direct perception.

But we have means to demonstrate to our senses the passage of air through our building-materials; we have only to lead the air which comes against some large surface of wall into and through a narrow tube. I will prove this to you by experiments; but you have often seen the same thing before, when you were looking at some piece of water which had some small in-and outflow. These may be in lively motion and driving mills, but on the whole surface the water seems to be completely at rest. But, if we do not see any water running in and out, we declare the whole to be stagnant, and we may be very much mistaken.

I have here a cylindrical piece of mortar, half lime, half sand, five inches by one and two-thirds. The cylinder has been covered all over with melted wax, which is inpermeable to air, with the exception of its two circular ends. You see this glass funnel, with a tube. I fix it on one circular end, where the mortar lies free, and make an air-tight connection by wax with the waxen coat of the cylinder. If I blow through the tube, the air must appear on the free mortar-end, provided the mortar is permeable to air. It has as yet no effect on the flame of this candle, because its velocity is not great enough. But if I fix a funnel on the other end of the cylinder, the air which has passed through the mortar can only escape through its narrow end, and there you see the flame sensibly deviating. You may even succeed in extinguishing it altogether. The velocity of the air in going through the tube must increase in proportion as the transverse section of the

tube is smaller than the mortar-surface, out of which the air escapes, exactly as with the water of the pond and its in and outflow. Now, when I dip the end of one tube into water, you see and hear the air which has passed through the mortar escape from the water. If you make a similar arrangement with a piece of wood, or a brick, you will see the same result.

Most kinds, also, of sandstone are so porous that water and air easily pass through them. Solid or quarried limestones are scarcely permeable to air, but, as they are mostly of irregular shapes, they require more mortar, and that is the reason why such walls are, after all, not so much more air-tight than walls made of regular bricks and thin layers of mortar. Observations have been taken of the average quantity of mortar used with different building-stones. We may suppose that, taking the wall as a whole, it is one-third with quarried lime, one-fourth with tufaceous lime, one-fifth to one-sixth with bricks, one-sixth to one-eighth with cubes of sandstone. In practice, then, the quantity of the mortar rises with the decrease of porosity in the building-stones, and assists in keeping the walls pervious to air to a certain degree.

It is self-evident that the quantity of air which passes through building-materials of a certain thickness must increase in proportion to the surface; two square feet must give passage to twice as much air as one square foot. I shall speak of ventilation in connection with this later on.

The effect of wetting porous materials is quite surprising. In proportion as the pores fill with water, they become impervious to air. The adhesion of water to stone and mortar is greater than that of air, by as much as water is heavier than air. It is not difficult to blow great volumes of air through dry mortar and dry bricks, but it requires a great exertion to drive a few drops of water through the same materials. You know this cylinder of mortar (see above). Instead of blowing air through it into water, I will suck the air out of it: you see now the water rise in the tub and wet the surface of the mortar. Now I'll try to blow again air through the mortar; I cannot, with all my exertions, because the pores of the mortar are filled with water.

This simple experiment lays bare the great hygienic disadvantage of wet walls; they are air-tight, not to speak of other injurious effects.

We all know that new houses are dreaded on account of their humidity. In some countries there are regulations by law, and new houses must be approved with respect to their dryness before they may be let. But the notions about the causes of their humidity, and the means of removing it, are very different and discordant. Allow me, therefore, to explain how water gets into the new house, and how it is to be got out of it.

I need not call to your mind the first steps in a building operation, and how soon a connection is made with some abundant source of water, and that a great deal of water is required for making the mortar, etc. Let us now try to come to an estimate of this quantity of water.

Suppose that 100,000 bricks were used for a building, each weighing ten pounds. A good brick can suck up more than ten per cent, of its weight in water, but we will put down at five per cent, what gets into it by the manipulations of the bricklayer. We will assume that the same amount of water is contained in the mortar, a quantity certainly much understated, although the mortar forms only about one-fifth of the walls: we have thus 100,000 pounds of water, equal to 10,000 gallons, which must have left the walls of the house before it becomes habitable.

The two principal ways in which wet or damp walls are injurious are: 1. By impeding ventilation and diffusion of gases, through their pores being closed up or narrowed by water; 2. By disturbing the heat-economy of our bodies. Damp Avails act as bodies abstracting heat in one direction; they absorb heat by their evaporation, and act like rooms which have not been warmed thoroughly; they are better conductors of heat than dry walls, just like wet garments, and considerably raise our heat-losses by a one-sided and increased radiation. Diseases which are known to be often caused by cold are particularly frequent in damp dwellings—rheumatism, catarrh, chronic Bright's disease, etc.

What can we do to get rid of that immense quantity, of these 10,000 gallons of water, before we remove into the new house? All this water—we cannot make it run off, we cannot squeeze it out, we cannot boil it away—it must take its leave in one way, a very safe but rather long one, that of spontaneous evaporation into and by the air.

The capacity of the air for receiving water depends on the different tension of the vapor at different temperatures, on the quantity of water already contained in the air flowing over a moist body, and finally on the velocity of that air. For the first two moments let us assume the average temperature of the year to be about 50° Fahr., and the average hygrometric condition of the air to be seventy-five per cent, of its full saturation. Under these conditions, one cubic foot of air can take up four grains of water, in the shape of vapor, but as it contains already seventy-five per cent, of these four grains, which amounts to three grains, it can only take up one additional grain. As often, then, as one grain is contained in the 10,000 gallons of water mentioned above, as many cubic feet of air must come in contact with the new walls, and become saturated with the water contained in them; or about 700,000,000 cubic feet of air are required to dry the building in question.

I will at once pass on to the consideration of a subject which some of you may be acquainted with already by experience—I mean the reappearance of damp in new buildings, which had seemed quite dry, after they had become inhabited. There appear damp spots on walls and in corners, the panes in the windows sweat, and the air becomes musty and oppressive. How does this water return, after the house has been declared and considered dry? Most people, because they see it only then, suppose that there is a new formation of water in the wall, or that it was set free by the presence of the new dwellers. Here, again, our sensuous perceptions mislead our judgment, and give us no clew as to the circumstances under which moisture in the walls becomes visible to our eyes and humidity produces a damp spot. I produce here a piece of brownish-yellow paper of somewhat indistinct tint. Where I wet it with water, the color appears more intense, darker even, just as if the water had been colored. Now the paper is, getting dry again, and the former appearance comes back.

Some of you may laugh at me for making such a trivial experiment, but I beg to ask, What is the reason of this action of the water? It takes place only on porous colors or colored surfaces which are porous, only on aquarelles or frescoes, scarcely on porcelain or glass paintings. If the water cannot penetrate into the color it cannot alter its appearance any more than that of a colorless, transparent glass.

Oil-paintings, when they are new or lately varnished, are in this respect like glass-paintings, but when they get older, some of the colors, by the longer action of the air, get dull, and then water has the same effect on them as on this paper. It gives them a fresher appearance till it has dried away. This is because all oil-colors become porous in the course of time.

For water to penetrate thus into the colors, we are entitled to assume that there must be free spaces within them to receive it, pores and interstices. These cannot have been vacua before, but must have contained air. This air in the painted surface is displaced by the water, and hence the difference in the optical effect. Air and water have different optical properties. In the first instance our colors—dry and dulled—are mixed with air; in the second instance with water. Water refracts, disperses, and reflects light quite differently from air; therefore it must have quite a different effect on colors when it gets admixed with them instead of air. The whole question has been more fully treated by me in a little treatise on oil-colors and the preservation of galleries; it may suffice here, and for the present, to know that damp spots on a wall can appear only when the pores are filled with water or some other transparent liquid. Our sensations have rightly taught us to associate the words dry and airy, damp and confined.

If we have moved into a new building too soon, we may be deceived by its appearance. It is quite possible that the walls have become sufficiently free from water and full of air for the colors of the papers and walls to appear mixed with air and free from all interstitial water; still, we are not entitled to suppose that all water has left the walls. A good deal may remain unnoticed, provided some air is lodged in the pores of the surface sufficient to produce the optical effect of real dryness.

How does it, then, happen that, on receiving their complement of inhabitants, the pores of the new walls become obstructed, partly or locally, by water? The ordinary explanation is completely erroneous, although it sounds quite scientific and rational, and has its place in books and lectures on chemistry. It is stated that it is the effect of carbonic acid on the hydrate of lime which remained in the mortar. Mortar is a very interesting object, and I regret that I cannot enter more fully into its nature and process of hardening. I'll tell you so much, that the burned and slaked lime used for its preparation is a compound of lime (oxidized calcium) and water, the above-mentioned hydrate of lime. This, by the action of the air, is changed into carbonate of lime. This change takes place at first very rapidly, and to the extent of about one-half, but then slower and slower, so that in very old masonry one finds frequently some of the original hydrate of lime. This is a perfectly dry substance, which yields none of its water to air which is dry and free from carbonic acid. When changed into carbonate of lime, the water, which as a hydrate it contained, chemically combined, is set free, while the lime and the carbonic acid combine. This water is commonly considered to produce the damp spots which appear here and there in new buildings. It has been imagined that the respiration of the new inmates increases the amount of carbonic acid in the air, and accelerates the process, setting free the water, which renders the wall damp and chokes up its pores.

This explanation is not based on any single direct observation made on the wall itself; it is nothing but a specious conclusion. Although hydrate of lime exposed to air which contains carbonic acid changes into carbonate of lime, no one has ever found it becoming moist. The liberation of the water, however much it may be accelerated in the indicated way, is unable to refill the pores of the wall, which were supposed to be already filled with air. To do this it would be necessary that the water in the hydrate should not have occupied any space, or that, when set free, it underwent such an expansion as water becoming a gaseous substance. All scientific analogies and observations protest against this. Changes of solid into liquid bodies take place without any considerable increase of volume; it is different with the transition of liquids into gases when the increase is very considerable.

It is only by the complete choking up of the pores by water, and the complete expulsion of the air from the surface of the wall, that the damp spots can be formed; and the freed water of the hydrate, which cannot fill a space which it had not filled while in its former combination, cannot do this. So the absorption of the carbonic acid is unable to produce the required increase of volume.

The fresh spots in new buildings can only arise from the precipitation of water from the air on the walls.

The inhabitants of a house give rise to a great amount of watery vapor, not only by the functions of their lungs and skin, but also by the numerous manipulations of the household, such as cooking, washing, cleaning, etc. If the air in the house is already saturated with water in proportion to its temperature, a small degree of cold in the wall is sufficient to produce a dew, a precipitation of water from the vapor, just as one sees it on window-panes. But the porous wall can imbibe a good deal, and in old buildings we may see the windows sweating profusely while the walls seem to remain dry. It may last a long time before a well-constructed wall or partition gives any sign. They go on condensing water till their pores are filled and all the air expelled—then, not slowly and gradually, but all at once, numerous damp spots make their appearance.

It is, therefore, clear why those youngsters of houses are so much more subject to damp spots than their brethren of more mature age. Their walls have lost just enough of the building-water to allow the air to occupy part of the pores; optically, they seem dry, but still very little water is required to choke up the pores here and there anew, and wherever this takes place the spots break out. The effect of a fire is very instructive; nothing produces damp spots so easily in a fresh building as the first fire, when doors and windows are well closed. The heat from the fire begins by heating the places nearest to it, and a good deal of water evaporates, so that the air in the room must come nearer its point of saturation. But at a distance from the fire, the walls being colder than the air, dew falls, and, if the pores still contain great quantities of the building-water, they soon begin to overflow.

Another proof that the water chemically combined with the hydrate of lime is not able to fill the pores when it becomes liquid lies in its proportionately small quantity. A house built, let us say, with 100,000 bricks, contains, at most, about 33,000 pounds of burned lime. This cannot combine with more than about 10,000 pounds of water in becoming a hydrate. By the time the mortar is hard and set, and the building becomes inhabited, probably one-half of the lime has become a carbonate, and there remain only 5,000 pounds of water in the remaining hydrate, which is five per cent, of the whole mass of 100,000 pounds of water which got into the new building during its erection. If, then, the other ninety-five per cent, of the building-water were gone, the five per cent., or even ten per cent., remaining, or formed by the change of hydrate into carbonate of lime, would not produce the optical phenomenon of dampness.

I have dwelt somewhat longer on this subject because it is indispensable for a correct view of the function of the wall: the removal into the open air of a great part of that watery vapor which develops itself in every human household. Our walls have to swallow a good deal of that vapor as water, and to pass it on through their body that it may evaporate on their outer surface. That is the reason why localities looking to the north, or shaded from the sun, are so much damper. This appears most clearly in unheated places, chiefly at the transition from winter to spring, when it is warmer outside than inside. We are glad to have once more the windows open to let the tepid spring air gladden the cold interior; but a good deal of water will soon be seen deposited on the walls and the objects within the rooms, which has to evaporate just as from a new building.

You are now well aware of the usefulness of porous building-materials; they alone can make dry dwellings. I cannot help thinking badly of all the substitutes for wood, brick, and mortar, which have been proposed, as zinc, iron, putty, etc. Perhaps the natural functions of the mortar-wall may one day be efficiently exercised by something else; for the present it has not been done, and will not be done as easily as many so-called practical people suppose.

Let me just relate to you a case, which shows that, without a correct view of the functions of walls, an apparently excellent plan may just produce the reverse of what was intended. In the neighborhood of iron-smelting works, the slag is often used for building-purposes. This material, associated with other stones, does very well. As it exists only in very irregular shapes, it requires large masses of mortar. and in our case this was believed to be undesirable. So it was decided to take only large regular pieces for the erection of a large workmen's dwelling, by which means the application of mortar could be reduced very much. It was a pleasure to see how quickly the building proceeded, and how much more quickly it got dry and habitable than other buildings, where irregular pieces and much mortar had been used. As soon as the workmen and their families began to live in the new building, the traces of damp began to show, and at last the house became the dampest in the whole establishment, and remained so. The thin layers or bands of mortar could not dispose well of the water which was deposited from the air in the house, and this was the worse, as the slag is not like brick and mortar, which suck the water up, but is a vitrified substance, on which water precipitates as on a window-pane.

But how are we to judge, in a given case, whether a house is sufficiently dry? No doubt, in every locality a practical experience establishes itself, founded on the knowledge of the usual material, the manner of building, and the climate. But if, as in some countries, some authority has to declare a house dry and habitable before it is to be let, there will be no end of disputes between this authority and the proprietor, because, after all, apart from the age of the building, the verdict of the experts will be given on their subjective view, not on definite and palpable signs. You know, already, that the absence of damp spots means very little. Feeling by the hand the temperature of the walls, knocking at them with a little hammer, are all of not much good. Not a bad plan is to get from different places in the house small pieces of mortar, and to have them examined as to their contents of evaporable water, which ought not to be more than five per cent, of the weight. But we may have fallen just on dry places only, and get considerably deceived. Direct and comparative hygrometric observations would be best, but the necessary preliminary researches for this kind of examination are still to be made.

But what is to be done if a new building is to be brought quickly and surely into a condition of dryness? I have been obliged to shake your belief in the one means which appeared to exist, the development of carbonic acid by burning charcoal in basins or open stoves. But I shall try to give you something in exchange for what I have taken from you. This something is nothing but an appeal to what we have learned above. There are no means of removing the water from a fresh building but by letting it evaporate into the air. This evaporation, you know, depends on the temperature, the humidity of the air, and its velocity.

Imagine to yourselves a moderately-sized room of 3,530 cubic feet, and the temperature and humidity of the air at the above given mean averages. As one cubic foot of such air is capable of taking up one additional grain of water, the air of the whole room will take up 3,530 grains, or about half a pound, of water. Should there be no change of the air, matters would remain so. But by every fresh 3,530 feet of air coming into the room another half-pound of water would be taken up, and so on. Suppose the change amounts to 353 cubic feet per hour: all the moisture we get rid of per hour would be only 353 grains per hour. But if we heat the room to 68° Fahr., for instance we increase the tension of the vapor, i. e., the capacity of the air for taking up water, from four to seven grains per cubic foot, so that each cubic foot of fresh air entering the room is capable of taking up seven instead of four that means four grains in addition to its original humidity In consequence of this increased capacity, the 353 cubic feet of air take up 1,412 instead of 353 grains of water. But by the increased difference of temperature between the room and the open air, ventilation rises from 353 to 2,100 cubic feet per hour, and in this way we get rid of more than twenty times as much water as if we left the room unheated.

All kinds of stoves and charcoal-dishes act only as sources of heat, and not as sources of carbonic acid. The only rational and efficient way is the heating of all the chimneys and stoves, and the continual ventilation of all the rooms. All other ways are of no use, or deceptive.

You have seen that the wall has its physiology, a life of its own. Perhaps you will no longer find it so strange that Master Quince introduces not only a pale moonshine and a rough lion, but also a "sweet and lovely wall" as a living and talking person. I had many things more to tell you about the wall, but I have still another subject of special importance before me—the change of the air in the house, or ventilation.

We have seen already, in speaking of our clothes, that the well being of our body requires a continuous current of air to flow round us, and for the same reason a flow of air must take place continually from the open air through our dwellings. It used to be a current belief that in the still air of our houses we were separated and shut off from the external air. You know that this error arises from our nerves and senses believing the air to be quite calm and motionless, although there may be some movement. We must be thankful to our Creator for this our error, else we should probably have ceased to exist. However anxious we may have been to shut ourselves up from the external air, we still remain in connection and intercourse with it. No house can have an atmosphere of its own; it has that by which it is surrounded, which travels and flows through it slower or quicker, while the house, and whatever exists and goes on in it, have no other power than to render this air more or less impure.

This pollution must not overstep a certain line, and this line depends on the proportion between the pollution and the change and volume of the air-current.

We change and pollute the air within our houses in two ways: 1. By admixture of substances which were not in the air when it came to us; and, 2. By changing its normal composition. Both are unavoidable, but there are limits, which must not be overstepped. The impurities may be in the nature of gases, or dust. We often become aware of them by our senses, by sight, by taste, mostly by smell. The last sense is exceedingly sensitive for many substances; for instance, traces of ethereal oils. Nothing is more wonderful than its acuteness with some savages and animals. If we consider the minuteness of the substances left by hunted game on the soil, which it scarcely touches in its flight, and how the dog detects them even a long time after, we cannot sufficiently admire such a performance of the sense of smell. Other substances make themselves known in other ways, sometimes by some physiological effect. Oxide of carbon, for instance—a gas which is generated from burning charcoal—is not perceived by any of our senses, but if it is present in air to the extent of a half per cent, only, it destroys human life after a while. A few grains of veratria, rubbed down into a powder, will set all the persons in the room sneezing. Other substances, as the products of distillation of fats, or the smoke of wood, irritate the membranes of the eyes. Other vapors and kinds of dust act on the taste; for instance, aloe-powder.

We rightly consider all air, which acts on our senses or our feelings differently from air in the open, to be polluted.

The second way in which we render the air impure on its journey through our houses is that of altering the quantities of its components. We deprive it of oxygen by our respiration, by the burning of lights and fires; we increase its carbonic acid and its water by the activity of our lungs and skin, and by numerous proceedings of the household.

All these pollutions and alterations are partly avoidable, partly unavoidable. Among the latter are those by our lungs and skin, because we cannot live without producing them. To the former belongs everything that from want of cleanliness, careless treatment of waste and refuse, passes into the air-current, the utilization of which ought to be the privilege of our skin and lungs. It is an inexcusable waste of ventilation, if it is directed against avoidable pollutions of the air, besides its being generally of not much use for this purpose. If I had a nuisance in my room, I should be a fool if I kept it there and trusted to stronger ventilation. The rational way is to do away with the pollutions, not to keep them and to fight them by ventilation. Without strict cleanliness in a house or public institution, all contrivances for ventilation will not do much good; the proper domain of ventilation begins where cleanliness, by rapid removal or careful shutting up of air-polluting substances, has done its best. It is only against the deterioration of the air by respiration and perspiration, which is beyond the control of cleanliness, that ventilation can direct its power and against this deterioration this power must be chiefly directed.

Let us now consider the different causes of the motion of the air. As air in motion is wind (ventus, Latin), ventilation is a better expression than "airing." Anything which disturbs the equilibrium of a body of air, produces motion in it. Its immobility supposes equality of temperature and specific gravity, and also of mixture in quantity and quality. Such conditions, as you may suppose, are seldom present, and absolute calmness is impossible. Different kinds of gases tend to intermix in every direction, even contrary to their specific weight, a process which is called diffusion; but this kind of motion is not in question when we speak of ventilation. Ventilation means the setting in motion of masses of air by mechanical pressure and the dislodging of whole bodies of air similarly composed, which, for that reason, are not subject to diffusion.

We produce ventilation by disturbing the equilibrium of the air in two ways: 1. By producing differences of temperature between two neighboring bodies of air, which are accessible to each other; 2. By mechanical pressure on or driving off the air in a certain direction. We cause the same motion in either way, but the first we call draught, the second wind; we call forth a draught by a chimney, we produce a wind by a fan, a fan-wheel, etc.

These two factors of change of the air are continually active in our houses, but to a very different extent at different times. Our houses stand in the open air, which is never quite calm; even if it appears so, there is still some imperceptible motion, some wind disposable for ventilation. Then our houses are either colder or warmer than the surrounding air. They act just like large chimneys. If they are colder, the air which comes in contact with them gets colder, and a downward air-current is produced; if they are warmer, the air gets warmer, and an ascending current is established.

It is evident that the intensity of the change must also depend on the way in which the house is shut up, on the size and number of its apertures, and the porosity of the materials it is built with.

For this reason a certain amount of ventilation is always taking place wthout any special arrangement for it, but its strength depends—1. On the amount of difference of temperature between outside and inside; 2. On the strength of the wind or air-motion in the open; 3. On the size of the apertures which are open to the change of the air. We may call the first two the air-motors, the last the air-mediator or janitor, door-keeper; to a certain degree, they can take each other's place. If there is not enough difference of temperature, as, for instance, in summer, the wind can act; if both together are too weak, opened doors and windows can help. In winter, when the difference of temperature between the in-and out-door air is considerable, small openings allow the passage of as much air as large openings during summer, because the difference of pressure is greater. When, in winter, we stay in an unheated room, whose temperature is only slightly above that of the outer air, ventilation is quite as weak as in summer; the air, if the windows remain closed, becomes quite as bad by our presence, and we ought to air the room as in summer; but we do not, because we want to protect ourselves against the outside cold. The dwellings of the lower classes present frequently, during the greater part of winter, this form of defective ventilation, which gets worse with the length of the cold season. In the beginning the walls are still dry and porous, and assist the ventilation, so far as the wind helps them to do so; but in proportion as they get colder, they increasingly condense water from the air of the house, and finally become so choked up with it that they allow no air to pass, as you have seen in our moistened piece of mortar. Bad doors and windows, unmended window-panes, remain often the only routes of ventilation. Poor people, in complaining of them, are not aware that they are the smaller of many evils, and a defect without which they might suffer still more.

Many of you, on hearing this, may be gratified by an unexpected personal satisfaction. Those who try to alleviate the poor man's winter by gifts of fuel not only procure for him the benefit of a warm room, but also of a better and purer air in this room. You may consider this as a scientific parable, showing that in each benevolent action there lies a further blessing, even if we had not intended it.

It follows, from these fundamental principles of ventilation, that a great mistake is sometimes made in large dormitories. In the morning the custom is to open the windows, and to let them remain open all day long, to be closed only just before bedtime. The poor sleepers fancy that they are sleeping all night in a pure air. Whoever has occasion to enter such a place in the morning, before rising-hour, starts back before this "pure air," which had only been renewed during the night partially and accidentally, and is so loaded with all kinds of animal emanations that it presses with all its power on the fresh comer. If there is no sufficient difference of temperature between outside and inside, a partial opening of the windows during a winter night is just as necessary as during a summer night, as far as regards the change of the air.

The bodies of the sleepers are certainly a small source of heat, and such large sleeping-places become somewhat warmed by the human heat flowing from the beds, but they can never be warmed through and through, so that the walls could become warmer. The water-vapor exhaled by the sleepers condenses against the walls, and goes on obstructing their pores till morning. A part of this water may evaporate during the time that the windows are kept open, but it will be only a part, and hence the frequent breaking out of damp spots in such dormitories in the course of the winter.

There was a belief that sleeping in the cold was a good thing; but I cannot find any facts proving this theory, particularly no comparative observations about the wholesomeness of heated and unheated dormitories. It would be safer to say that experience proves that sleeping in the cold does not, generally, do harm. If a single person sleeps in a large cold room with shut doors and windows, it will do him no harm when he has a good bed. One person cannot deteriorate the air of an unventilated space as much as two or more. The bed is a garment, an apparatus, which is of great use for our heat-economy; it prevents our feeling cold even in the coldest dormitory, but the bed is no ventilating apparatus, and ventilation must be provided for in another way. He that wants to sleep safely in the cold must have a good bed and a large space, or bad windows and doors, or very porous walls, or he must keep his windows partly open in winter as well as in summer.

You have probably now the desire to hear from me how much air or ventilation a person wants in a stated time. After you have all the while heard from me that everything is full of air, that air penetrates everywhere, and that it is extremely difficult to prevent its passage, many among you will ask: "What need is there of special contrivances, if the air passes through each brick, through mortar, through wood? Would it not be rather desirable to protect ourselves against this universal aggression of the air?"

It is with air as with all things which we must have—as with money, of which we must not only have some, but sufficient—one must have as much as one requires. Some money is, after all, in everybody's possession, even that of the poorest beggar.

Till some time ago, ventilation was chiefly considered in its qualitative aspect: we wanted change of the air, and were satisfied if there was one aperture for it to go out, and another to come in. The question about the quantity of this air was never put; if it had been known how much was really wanted, and how it was to be procured, that amount of ventilation which was often paraded would have appeared beggarly. It is only during the last twenty years that we have acquired clear ideas on this subject.

We deteriorate the air of a closed space inevitably by using it for the maintenance of our respiration and perspiration. To which degree, then, may we alter or pollute by our emanations the air of a closed space, without going so far as to injure our health? This leads us to another preliminary question: What standard have we for measuring the deterioration of the air?

At all times people have been in the habit of making some estimate of the pollution of the air by the smell imparted to it by the respiration and emanations of the persons staying in it. This estimate is of the same value as that we have spoken of when on the subject of the water in the walls. The smell of a certain air need not be in any kind of proportion to the use which has been made of it already for the purposes of respiration and evaporation. Besides, smelling is a very subjective sensation, of very different excitability in different persons. Although generally a certain rule for judging the air of a room may be based on its smell, the decision, in doubtful cases, will always be a subjective one. It would be a different thing if we could lay hold of the smelling particles in the air of the room, and measure or weigh them, and compare them with the volume of air they were taken from; but we have no method of doing this; everything is left to our noses.

For this reason I considered it indispensable to look about for some means which would make us independent of our subjective estimate. I started from the excretion of carbonic acid, as it takes place from the living human body; its quantity in the air can be ascertained easily and accurately. There is some in the open air, although very little; the question was, therefore, to find out its increase in a number of inhabited rooms, with notoriously good and notoriously bad air, and to draw a comparison. The correctness of this proceeding depends on the supposition that there are no other sources of carbonic acid but the inmates, that there are no burning flames, or tobacco-smoking, etc. I will not say that I consider the detected carbonic acid as the principal drawback to such air; it is, in my mind, the measure only for all the other alterations which take place in the air simultaneously and proportionately, in consequence of respiration and perspiration; its increase shows to what degree the existing air has been already in the lungs of the persons present. All other functions in which the air participates keep in some proportion to the respiration.

A series of examinations have resulted in the conviction that one volume of carbonic acid in 1,000 volumes of room-air indicates the limits which divide good from bad air. This is now generally adopted and practically proved, always provided that man is the only source of carbonic acid in the space in question.

Suppose there is a known source of carbonic acid: the determining the amount of it in a room can also be used for measuring another element, which would otherwise defy calculation—I mean the amount of ventilation of a closed space of definite construction. Imagine to yourselves a room with its walls, windows, and doors, its numberless penetrable places through which the air holds ingress and egress. It is impossible to measure the velocity of the air at each crack, to measure each little hole, the diameter of each pore, even if one had the means of measuring such minute velocities and sections; yet still we should like to know how much air changes in a given space, and under different external circumstances. The only way appeared to me to be to mix the air of the room in question with carbonic acid to a certain degree, then to break off this mixing, and to observe the decrease of the acid in proportion to the air in definite times. Knowing the amount of the acid in the external atmosphere, we can calculate how much of the latter must go on mixing itself with the room-air, to which carbonic acid has been added, in order that the proportion of the acid may decrease by so and so much in a definite time. The action of diffusion or absorption may generally be left out of consideration in this calculation. I do not consider this method to be absolutely correct, but I have found it quite satisfactory when a building was a few years old, and quite dry. At all events, until a better method has been found, we must keep to this one, even if it were still less complete than it is.

By researches which are too complicated to lie explained in a popular lecture, it has been found that the ventilation of the same room or space, when the doors and windows are shut, undergoes considerable and definite alterations under different circumstances. Ventilation has been found to be much greater than had been supposed before. On an average, in spaces in which the air kept good, there existed a ventilation of more than 2,100 cubic feet per head and hour. It is known that a person does not inhale and exhale more than eighteen cubic feet of air per hour, and so it was thought that 2,100 cubic feet per hour was a ridiculously large quantity for one person.

But it has been shown, first in France, not by calculation, but quite empirically by simple experimenting, that this quantity of ventilation is not more than is absolutely indispensable. After the epidemic of cholera of 1848, the erection of a model hospital in the Faubourg Poissonnière was decided upon, and the Hôpital la Riboisière was erected, which was to be furnished with artificial ventilation. The quantity of air which was required from the ventilating apparatus was stated in the plan. It was believed that the demands put under Nos. 4, 5, and 7, of the plan for ventilation, were extraordinarily large:

4. Continuous ventilation of warm air in winter and cold air in summer at least 700 cubic feet per hour and bed in the large wards.

5. Ventilation during the day only at 350 cubic feet per bed in the rooms of the corresponding pavilion.

7. The ventilating apparatus must have a surplus of strength, in order to be able to produce in all or some wards a ventilation double that stated.

The air was partly propelled by fan-wheels, partly by ventilating flues. It flowed to and fro through pipes in the wards, and its velocity could be measured easily by anemometers.

In preliminary experiments, a ventilation of 350 cubic feet per bed and hour was tried, but the air was found already by the smell to be so bad that the authorities congratulated themselves in having provided for double the strength. This was now tried, but with the same result, and it was a comfort to know that, for extraordinary cases, another 700 cubic feet per bed and hour could be obtained; but then also the state of the air was anything but desirable. It was only with 2,120 cubic feet that the medical and other authorities found themselves satisfied.

At present the demands for ventilation in France sound very different from what they were about twenty years ago. They are now per hour and person:

Hospitals for ordinary cases 2,120—2,470 cubic feet
"wounded 3,530 "
"epidemics 5,300 "
Prisons 1,766 "
Workshops, ordinary 2,120 "
"unhealthy 3,530 "
Barracks, day 1,060 "
"night 1,410—1,765 "
Theatres "" "
Large rooms for long meetings 2,120 "
""shorter" 1,060 "
Schools for children   424—   530 "
"adults   880—1,060 "

Such are the changes of times.

Now the many crevices, holes, and pores in our dwellings will no longer be considered by you as unlimited means of change of the air, since you know how large that change has to be; you will rather feel anxious whence to procure such enormous quantities when you sit quietly within your four walls where you do not feel the least draught, where no curtain moves, and a feather lies quietly on the floor. This sensation of calm we owe to the insensibility of our nerves—and yet the air moves.

In order to give you some idea of the influences of differences of temperatm-e of more or less well-shutting doors and windows, of a fire in a stove opening into the room, and of the partial opening of a window, I will give you shortly the results I obtained with the aid of the carbonic-acid measurement. The room had brick walls, and its size was 2,650 cubic feet.

With a difference of temperature of 34° Fahr. (66° in-and 32° out-side), the contents of the room changed once in one hour, equal to 2,650 cubic feet.

With the same difference, but a good fire in the stove, whose communication with the chimney was made as free as possible, the change of the air rose to 3,320 cubic feet, or about twenty-five per cent. When all openings, crevices in windows and doors, were thoroughly pasted up, there was still a change of 1,060 cubic feet per hour, or a fall of twenty-eight per cent. With a difference of temperature of 71° in-and 64° outside, the change amounted to 780 cubic feet only per hour. When opening a window of eight square feet, the change rose to 1,060 cubic feet per hour. These quantities are instructive. They show that a difference of temperature of 34 with carefully-shut openings and crevices is of greater influence than large communications with the outer air at a small difference of temperature.

The roaring fire and the draught of the stove produced only an increase of 700 cubic feet—one-third only of the necessary ventilation per head. I have examined a number of stoves opening into the room for the quantity of air which they abstract while the fire burns. The anemometer showed that it was never more than 3,105 cubic feet. Large wards in hospitals, schools, etc., heated by one open fireplace or stove, are sometimes wrongly believed to be well ventilated, because one perceives the air rushing into the same. But the main point is to know the quantity of required air and the quantity of outgoing air.

The free wall of a room of mine has been examined for its ventilating power. The room contained 2,650 cubic feet, and at 9.5° Fahr. difference of temperature between outside and inside, the spontaneous ventilation through each square yard amounted to about seven cubic feet, or forty-three gallons per hour.

Mãrker and Schultze, in their researches on the spontaneous ventilation of stables, have found for one square yard of a free wall, at 9.5° Fahr. difference of temperature, that the spontaneous ventilation amounted per hour—

With walls of sandstone to 4.7 cubic feet.
""quarried limestone " 6.5 ""
""brick " 7.9 ""
""tufaceous limestone " 10.1 ""
""mud " 14.4 ""

Domestic animals, according to Mãrker, require a proportionately smaller change of the air than man. Stable-air may contain up to three per mil. carbonic acid. While man's ventilation requires 2,100 cubic feet per hour, 1,050 are sufficient for full-grown cattle, although their bodies and consumption of air are so much larger. The ventilation of stables depends chiefly on the size and porosity of their free walls. It has been found that the 1,050 cubic feet mentioned above were furnished by—

21.16 square feet of a free wall of sandstone.
15.33 """" quarried limestone.
12.6   """" brick.
9 """" tufaceous limestone.
7 """" mud.

A stable built up of mud can therefore shelter many more animals than one built of sandstone, etc. "As the strength of the natural ventilation of a stable does not depend on the cubic space of the stable, but on the extent of its ventilating walls, it follows that in a small stable a proportionately greater ventilation takes place than in a greater one, because for each animal there is more ventilating surface with equal cubic space."

This proposition naturally applies also to human habitations. The air will be better in a small family house than in large barracks; better in a cellular than in a common prison, where the day and night wards are large but crowded.

The question arises, "What is to be done in all cases in which the natural ventilation of the inhabited spaces proves insufficient, and allows the carbonic acid to become more than one per 1,000?" I might tell you now of the different systems of ventilation, the contrivances and apparatus belonging to them; but this is not feasible without models and designs. And, after all, there would be no new principles or natural laws to acquaint you with. I believe I have made you sufficiently aware of the fundamental facts and conditions as to the change of the air in our dwellings, so far even that you are now able to judge for yourselves whether a certain plan for ventilation is rational or not. We have no other motors for changing the air, but differences of temperature and motion of the air, which we can call forth either by heat or by the motion of wind-fans—or which we must make use of as far as they are preexisting in the atmosphere surrounding the house. By these two means we can produce certain perturbations in the equilibrium of the air-columns, and through this certain degrees of velocity in the motion of the air.

If we know the transverse section of the inlets and outlets, we have only to multiply their surface by the' velocity of the air, and this will give the cubic quantity of the air which flows through the channels in a certain time. If we know the required quantity of air and divide it by the transverse section of the channels, we get at the velocity of the air in the channels. We ought not to establish a greater velocity than nine feet per second; it is better to enlarge the channels. These quantities must then be compared with the air required by each person, a quantity with which you are now acquainted.

If you take up the question of artificial ventilation in its quantitative aspect, you protect yourself at once against a series of errors into which else you easily fall. Our ordinary dwelling-houses need not be ventilated artificially; we ought never so to crowd them that the natural means of ventilation, as difference of temperature, motion of the air in the open, dry and porous walls, and temporary assistance by the architectural openings, are insufficient to keep undeteriorated what is most essential for our health. With these means there must go hand-in-hand the greatest cleanliness in all parts of the house, and abstention from all superfluous and avoidable pollution of the air of the house.

Before concluding, I am desirous of considering with you an expression which is in general use, but the frequent cause of wrong views about the change of the air. I mean the word draught. All kinds of complaints are habitually ascribed to it, and the danger of draughts is one of the few hygienic principles which have become thoroughly popular. Perhaps this was not all profit, because with many people ventilation and draught are synonymous; they are afraid of a draught coming from an open window, an open door, and find themselves in collision with ventilation.

There is certainly and frequently danger in being exposed to a draught—a danger which has, perhaps, been over-estimated, because men have an irresistible desire to fix a certain cause for a certain evil. All collision is avoided if the proper meanings of ventilation and draught are thoroughly understood.

Ventilation is the necessary change of the air in a closed space, at which the velocity of the air is still taken for a complete stillness, and its motion takes place all round our body. It must not be more than a little above nineteen inches per second.

Draught is a one-sided cooling of the body, or some part of it, frequently caused by a corresponding motion of cold air, but also in other ways, as by increased one-sided radiation. The danger is, in the first instance, the local perturbation in our heat-economy, which has partly local consequences, but also and chiefly disorders the nerves, acting on the calibre of our blood-vessels, our vaso-motor nerves, which have to regulate the outflow of our heat. When we are in the open, and the air is in more motion than the air of a draught, we speak of wind, etc., but seldom of draught, because the whole air-current flows equally all round us, just as in a well-ventilated room, only with greater velocity.

The vaso-motor nerves, regulating the circulation in our skin, are beyond our control, and we cannot bid them to defend us simply at the place attacked by the draught. They know only how to serve our heat-economy when the outflow of heat from our bodies is equal, or nearly so, on all sides. They misunderstand the local irritation for one spread over the whole surface, and act at once on this error. If one perspires and goes to the window with bared neck or chest, one feels a shiver not only there but all over the body, and the perspiration becomes suppressed accordingly. The blood which at the time filled the blood-vessels of the glowing skin is displaced by the contraction of its channels; but by the misunderstanding of the vaso-motor nerves it is driven not only from the exposed parts but from the whole surface toward the internal parts. If one or some of them are in some state of weakness, danger or bad consequences cannot fail. It is the same thing as with a large quantity of cold water taken in too quickly when the body is heated. A draught, then, is injurious only in so far as it causes perturbations in our heat-economy, and as these perturbations can be caused in different ways we often accuse the draught wrongly.

We hear often, "I don't like sitting near this window, close to this wall," and so on; "there is always a slight draught coming from there." We fancy that we feel the draught, the motion of a wind, but it is mostly increase of one-sided heat-loss by radiation toward the cold place. People generally believe, rather, that the wind comes through the wall But the velocity of such a wind is too small to be felt as air in motion and a piece of carpet fixed to the suspected wall does away with the supposed draught. It could, therefore, not be caused by the air-rush through the wall, because the carpet is many times more permeable to air than the wall.

I hope, in future, ventilation and draught will be to your mind two distinct things.

  1. Abridged and translated by Augustus Hess, M.D., member of the Royal College of Physicians, London, etc.
  2. In England, owing to the manner of building, the smaller size of the houses, the open fireplaces, and the badly-fitting windows and doors, we suffer less from defective ventilation than in Germany; and, although some other domestic arrangements, though far from faultless, are superior to those usually met with in Germany, nevertheless, the general laws are the same, and ought to be generally understood. Translator.