Popular Science Monthly/Volume 24/December 1883/The Habitation and the Atmosphere
By M. R. RADAU.
IN a former article we endeavored to elucidate some of the principles which have been developed from the later researches and experiments on the relations of our clothing with the atmosphere (see "Popular Science Monthly," October, 1883). The house, also, may be regarded as a kind of clothing, as a large and ample garment, designed to regulate our relations with the surrounding medium, and to deliver us from its tyranny, but not to isolate us. It ought not to deprive us of air, though that point is too often forgotten. Fortunately, no voluntary prison is so tightly calked up that air from out-of-doors does not find entrance without our perceiving it. The fact that water will readily penetrate a wall or ceiling is known to all, for they can see the spots it makes; but the air that passes through walls is not seen, and so we imagine that it does not penetrate them. This is a mistake. Walls would not prevent us from being in communication with the outside air, even if no cracks were left around the doors and windows. If water can find a way through them, what is to hinder a subtile gas from doing the same? The porosity of walls is very far from being an evil; and we shall shortly see that it is necessary to prevent houses being damp.
A very simple experiment by Dr. Pettenkofer illustrates the permeability of building materials. He took a cylinder of dry mortar twelve millimetres (4·7 inches) long and one third as thick, and waxed all of it except the ends, in which he fastened two glass funnels, one of which was extended by an India-rubber tube, while the other terminated in a very fine orifice. Blowing through the India-rubber tube, he was able to drive the air through the cylinder with force enough to extinguish a candle at the other extremity. Similar results may be obtained with wood and such varieties of stones as allow air to pass through them without difficulty; while other stones, like compact limestones, are hardly permeable.
All materials become impermeable to the air when they are wet. The experiment with the cylinder of mortar will not be successful if the mortar is moistened. It has also been found less easy to drive moisture through bricks and mortar than to make air pass through them; only a few drops of the liquid can be made to appear on the free surface. Water is therefore not easy to dislodge from the pores it has occupied, and is at most removed very slowly by evaporation. But, when water stops the pores, it prevents the air from circulating through them—a mischievous effect upon the permeability of building materials, which is more perceptible in proportion as their grain is finer and more compact.
In ordinary weather, and when they are dry, walls perspire. They are continually traversed by feeble atmospheric currents, which renew the air of closed rooms and rid it of the moisture with which it is loaded. The atmosphere of a house is saturated with moisture by the respiration and perspiration of its inmates, and by the water daily used in housekeeping, even if we do not take account of the dew that is deposited whenever some air from without gets into cold rooms. This moisture, which is always undergoing renewal, ought to be absorbed by the walls, to be evaporated from the outside, under the action of the sun and wind. For this reason it is well for building materials to be porous and permeable, and for them to interpose no obstacle to the circulation of the air which is depended upon to promote evaporation. This remark is especially applicable in the North, where the windows can not always be wide open; it is perhaps of less importance in the South.
The moisture which the walls receive from the exterior atmosphere, from fogs and rain, generally disappears quickly enough under the operation of the winds that constantly lick the surface of the house. But the moisture that comes from within, which is deposited on the walls of poorly ventilated rooms, passes away with difficulty when the walls are not porous. Even the heating apparatus only causes it to change its place, by leaving the surfaces that become warmed and settling farther away where the heat has not yet reached. Inconveniences from interior moisture are especially sensible in newly built houses, where the mortar still contains a large proportion of water, and in ground-floors built on a damp soil, which become impregnated by capillarity. The water stops up the invisible channels through which the air should circulate, and the wall remains damp notwithstanding the evaporation that takes place at the surface, to the great harm of the inmates. Like wet clothes, damp walls are healthy because the water they contain increases their conductibility, and, consequently, the flow of heat from within outward; and also because evaporation absorbs or neutralizes much heat. M. Bouchardat, remarking in his "Treatise on Hygiene" on the exposure to which the tenement population are subjected in wind-penetrated Mansard-roofs and in damp basements, adds that the commissioners of unhealthy dwellings are wrong when they rank overcrowding and uncleanliness among the worst sources of danger.
Dr. Pettenkofer calculates that a house having a cellar and basement and two stories of five rooms and a kitchen each, would take 800,000 kilogrammes of bricks, and that these would hold about 40,000 kilogrammes of water. The mortar, although less bulky, would hold as much more water. Thus, the entire masonry would hold, in a house just built, 80,000 kilogrammes or eighty cubic metres of water—a quantity which it is by no means easy to drive out. Among the various means that have been devised for quickly drying the walls of newly-built houses preparatory to tenants moving in, only those can be of real effect that depend on the employment of heat combined with an active aeration. The question is wholly one of promoting ventilation. The lower the temperature, the greater the quantity of air that is needed. At 50° Fahr. a cubic metre of air, which may be already supposed to be three fourths saturated, contains seven grammes of vapor, and is only capable of receiving a little more than two grammes more. Thus, nearly 40,000,000 cubic metres of air at 50° will be needed to absorb the 80,000 kilogrammes of water in the masonry. A moderate wind might, it is true, bring this volume of air in contact with the exposed surface in the course of twenty-four hours; but it is evident that the moisture can not be carried off any faster than it can get through the thickness of the wall to the outer surface; and, when this has to be done, the time required for a more or less complete desiccation would be very long. A suitable degree of heating would greatly hasten the drying, provided the air were continually renewed. If, for example, the temperature of the room were raised to 68° Fahr., the effect—depending partly on the increased capacity of the air to absorb vapor, and partly on the greater rapidity of ventilation—would be five or six times as great.
Aëration is thus the sovereign remedy for the moisture of dwelling-houses, and it is favored by the use of porous materials. Viewed with respect to this point, direct determinations of the porosity, permeability, and hygroscopicity of different building materials are of great interest. Messrs. F. and E. Putzeys, in their work on "Hygiene in the Building of Private Houses," have compiled nearly all that has been published on this subject. It appears from their tables that, in the stones most usually employed, the pores occupy an important fraction of the whole volume. According to Hunt, the decimal of porosity is from 0·07 to 0·20 for some sandstones, from 0·06 to 0·14 for various dolomites, and 0·30 for the soft Caen limestone and Maltese sandstone. These figures do not, however, permit us to predict the relative permeability of walls into which the stone in question may enter, for that will depend as essentially on the proportion of mortar used and the kind of wash or plaster that is put over the stones, as on the kind of stone employed. It must, then, be determined by direct experiments. These are not wanting. Märker has shown that Walls of brick let more air through than walls of cut sandstone. Arranged in the order of increasing permeability, the building materials here mentioned would stand—sandstones, rough stones, limestones, brick, calcareous tufa, and adobe. Adobe has been found to be twice as permeable as burned brick, having a porosity of sixty per cent, while brick has only twenty-five per cent, by volume. Mr. Lang has made more complete researches on the co-efficient of permeability of different materials, and puts calcareous tufa at the head of his table. Then follow, in the order of decrease, bricks of slag, pine-wood, mortar, béton, hand-made bricks, green sandstone, molded plaster, oak-wood, and enameled bricks. Plaster is extremely compact, and little favorable to natural ventilation.
Paints, washes, and paper-hangings diminish the permeability of walls. The following surfaces are mentioned by Lang, in the order of their increasing effects: whitewash, mastic, glazed papers, common papers, and oil-colors. Common papers are more impermeable than glazed papers, according to Messrs. Putzeys, on account of the greater quantity of starch with which they are impregnated.
Indispensable as is the renewal of the air as a means of preventing moisture in dwellings, it is still more so as a precaution against impurities of every kind that would finally make the atmosphere unfit for respiration. It is, then, important to learn by what sign we may know when an atmosphere is vitiated, and what is the volume of air which a man requires for free breathing in a close room. Normal air, according to the mean of the results of five years of observations at the observatory of Mont Souris, contains about three ten-thousandths by volume of carbonic acid. Immense quantities of this gas are, however, produced in cities by the respiration of the inhabitants and by the fires, but the whole is so rapidly removed by the winds that the atmosphere is not sensibly vitiated by it and it is not necessary to estimate the proportion of carbonic acid, even in the most densely crowded localities, at more than four ten-thousandths.
In an occupied inclosure, like a sleeping-room, a school-room, or a public assembly-hall, the air undergoes a progressive change through the consumption of oxygen and by exhalations from the lungs and the skins of the people; and, unless a sufficient ventilation is kept up, it will in time become unfit for respiration. This will be the case when the impurities with which the atmosphere is charged become perceptible to the smell and provoke the uneasiness which is usually attributed to a close atmosphere. It is generally agreed that this condition is reached when the proportion of carbonic acid approaches one thousandth. Observation shows, in fact, that the proportion of carbonic acid increases in the same degree as the insalubrity of the air, and may, up to a certain point, afford a measure of it; but the inconvenience we suffer from bad air is in reality attributable rather to the putrescible organic products of respiration and transpiration which it contains. According to Péclet, the air driven out from the ventilating chimneys of crowded rooms exhales an odor so noxious that it can not be borne with safety, even for a short time. According to some chemists, the disagreeable odor that characterizes close air is due to a particular substance possessing an alkaline reaction and the property of giving off ammonia, which escapes from the lungs. The real culprits are these miasms which affect the smell. The carbonic acid, which is comparatively an inoffensive gas, only indicates the change the air has undergone. The experiments of MM. Regnault and Reizet go to show that an animal can live in an atmosphere containing seven hundredths of carbonic acid, provided the proportion of oxygen is maintained at twenty-one hundredths. Animals have been observed to perish in a tight inclosure even when the carbonic acid is eliminated as fast as it is formed, and the lost oxygen is restored; and Mantegazza has shown that if two birds are placed under two different bell-glasses, and the carbonic acid formed by one is absorbed by quicklime, and the organic matter exhaled by the other is taken up by animal charcoal, the latter bird will survive considerably longer than the former. We add that Dr. Pettenkofer has been able to breathe for several hours, without inconvenience, air containing one hundredth of carbonic acid developed, not by respiration, but by a chemical process. These facts indicate that the few thousandths of carbonic acid diffused in it are not the cause of the effects produced by an atmosphere vitiated by respiration. The oxygen content diminishes in nearly the same proportion as carbonic acid is developed; but the effects produced by "close air" can not be explained by the deficiency—say of one per cent—of oxygen; that may be remedied in part by more active breathing.
Carbonic acid has sometimes been wrongfully charged with effects which were really due to a small proportion of carbonic oxide, a product of imperfect combustion and of the reduction of carbonic acid. Carbonic oxide is a deadly poison, and destroys the red globules of the blood. To its disengagement may be attributed the unhealthy effects of cast-iron stoves, effects from which sheet-iron stoves, which are not pervious to it, are free; and it is one of the products of the combustion of poor illuminating gas. It is, nevertheless, customary to measure the degree of insalubrity which any atmospheric medium has reached by the quantity of carbonic acid it contains. This is found to increase rapidly in school-rooms, hospital-wards, and assembly-rooms of all kinds, but not nearly so rapidly, unless the room is extremely close, as the gas is actually developed by the life-processes of the inhabitants of the rooms. This fact indicates that, even in rooms regarded as close, a considerable renewal of air is all the time going on by natural or spontaneous ventilation.
Dr. Pettenkofer has made an ingenious use of the estimation of the proportions of carbonic acid to measure the spontaneous ventilation, or the speed with which the air gradually renews itself in rooms. It is sufficient for this purpose to develop artificially in a room an exactly ascertained quantity of the gas, and to determine by repeated analyses the quantity of acid that disappears in a certain time. The method is a good one, provided there is no opportunity for the acid to be absorbed by fresh mortar. By gauging in this manner the ventilation of a number of places, and then observing in the same places the degree of alteration in the atmosphere resulting from the presence of a given number of persons, Dr. Pettenkofer found that the atmosphere remained of a satisfactory quality when it was renewed at the rate of sixty cubic metres an hour per head. The proportion of carbonic acid continued under these conditions to be less than a thousandth. Experiments were made in a room with brick walls, and having a capacity of seventy-five cubic metres. On the first day when the temperature was 66° in the room and below the freezing-point out-of-doors—the difference being nearly 36°—the rate of change (seventy-four cubic metres) was sufficient to renew all the air in the room in an hour; with a good fire in the stove, the rate of ventilation was raised to ninety-four cubic metres an hour. With paper pasted over the joints of the doors and windows, it fell to fifty-four cubic metres. On another day, when the difference between the inner temperature and that out-of-doors was about seven degrees, the rate of ventilation was only twenty-two cubic metres an hour; and with a window half open it was only increased to forty-two cubic metres; thus an opening of eighty square decimetres was of less effect upon ventilation than the simple transpiration through the walls assisted by a difference of about 36 between the outer and inner temperatures. A calculation based on these experiments indicates that a difference in temperature of 1° C. (1·8° Fahr.) causes to pass every hour about two hundred and forty-five litres of air for each square metre of exposed wall-surface.
The question of the volume of air needed by a man for free respiration is a complex one, on which hygienists do not readily agree. The answer to it must depend, not only on the exterior conditions in view, but also upon the limit of variation, or tolerance, which is regarded as admissible in the normal composition of the air. In a room hermetically closed, where the volume of available air is limited by the capacity of the in closure, the proportion of carbonic acid will soon reach the one thousandth, which we have adopted as the tolerable limit; and the more speedily as the size of the room is diminished, the more tardily as it is enlarged. The volume of air required will also evidently be proportioned to the time the man stays in the room. Assuming that about twenty litres of carbonic acid are exhaled in an hour from the lungs of an adult man, we find that he will require about thirty-three cubic metres of fresh air every hour; for this quantity of air already has a normal content of thirteen litres of carbonic acid; and the addition to this of the twenty litres exhaled will bring up the whole amount to thirty-three litres, or the one-thousandth part of the volume of air, which we have accepted as the tolerable limit. Consequently the space a person must have, if he is to live in a really close room for an hour, is thirty-three cubic metres; if he is to live there two hours, sixty-six cubic metres. More will be needed if lamps or gas-lights are kept burning in the room, for a candle in burning will consume as much oxygen as a man; but the carbonic acid produced by combustion is not so dangerous as are the exhalations from a living being. The case of a perfectly close room will, however, never be realized; for, however tightly we may close the doors and windows, the air will always get in through some crack, and, if there are no cracks, it will penetrate through the walls. The most thoroughly calked room is not proof against the natural ventilation that results from inequalities of temperature. Houses are great centers of draughts in cold weather, and are permeated by a spontaneous ventilation that is dependent at once on the degree to which the outer atmosphere is agitated, on the number and sizes of the doors and windows, on the condition of the chimneys, and lastly on the permeability of the walls. It may be increased by a suitable distribution of ventilators, and is aided by the draught of the chimneys when fires are kindled in them; but fires may be regarded as artificial means of ventilation. These agencies of natural ventilation diminish in a notable degree the danger of the air within houses stagnating, and will always prevent its becoming vitiated to the extent that might otherwise be apprehended from the causes of contamination which we have reviewed. Their effect should be taken account of in estimating what extent of artificial ventilation may be required; otherwise, we might make exaggerated provisions for it.
When an inclosure containing a given number of persons is subjected to a regular ventilation, there is established, at the end of a certain time, a permanent régime; the adulteration of the air, having reached a certain limit, does not vary any more, the noxious gases being eliminated as fast as they are developed. The proportion of carbonic acid is from that time constant; we obtain it simply by assuming that the acid disengaged is distributed through the volume of air introduced by the ventilation. This proportion-limit is, then, independent of the disposable cubic space. A ration of forty cubic metres of air, for example, with a production of twenty litres of carbonic acid, to which are added the sixteen litres of acid contained in the forty cubic metres of fresh air, gives the proportion of 0·0009, whatever may be otherwise the disposable space. The capacity of the inclosure plays no other part than that of delaying the moment when the constant régime is established; the space acts as a reservoir which is gradually filled till it contains the same proportion of acid as the current of air that traverses it; but, once saturated, it intervenes no more in the course of the phenomenon. The advantage of a considerable cubic space consists, then, chiefly in the fact that it retards the approach of the moment when the alteration of the air attains the limit which it will not pass. This consideration becomes of some importance in fixing the size of rooms that are to be occupied only for a definite number of hours at a time; for it will be always possible to arrange matters so that the proportion-limit shall not be reached before the end of the contemplated time.
Let us suppose, for example, that the ventilation can supply six cubic metres of fresh air per person per hour. This is the ration of air which, according to Péclet, might be sufficient in case of extremity, because six cubic metres of air, half saturated at 60°, can absorb the thirty-five or forty grammes of vapor given out by transpiration. The fresh air containing already a proportion of 0·0004 of carbonic acid, to which respiration adds 0*0033, we find that the proportion limit will be 0*0037. This limit will be almost reached and the regime will be constant when the air has been renewed three times, for the proportion of air will then exceed 0·0035. If the allotted space is only one cubic metre, as we know happens sometimes to be the case in theatres and other assembly-halls, a half an hour will be long enough to bring about this state of things; if the cubic space is increased to ten cubic metres, five hours will be required, and ten hours if it is increased to twenty cubic metres, to reach the same degree of alteration. Such, then, would be the effect of a ventilation at the rate of six cubic metres an hour, according to the capacity of the building. By raising the ration of air to thirty cubic metres, the proportion-limit becomes 0·0011, and we may assume that this has been reached when the air has been renewed twice (the real proportion being then 0·0010). This will happen at the end of four minutes in a space of one cubic metre, after forty minutes in ten cubic metres, etc. But the prolongation of time obtained under these circumstances is not of the same importance as in the preceding case, for the limit of 0·001 is a characteristic of respirable air. With so energetic a ventilation as this, the consideration of cubic space becomes a minor affair; but it is of great importance when the only dependence is upon natural ventilation, for that is greatly facilitated by any increase of the extent of exposed surfaces, and of doors and windows. We should also keep in view that a like quantity of air will more readily traverse a large than a small space without producing inconvenient currents; and that the air in a large space requires less frequent renewal, and does not have to be kept in as rapid motion. Natural ventilation, which is uniform and almost insensible, must not be confounded with draughts and currents of air, with the injurious effects of which all are acquainted.The rules as to the amount of space that should be allowed in connection with natural ventilation are various and indefinite. Aëration from this source can not always, however, be depended upon; and even the opening of windows on opposite sides of an apartment frequently fails to produce the changes of air that are needed. General Morin, who has distinguished himself as an apostle of ventilation, and who made numerous experiments bearing upon the subject, has given the following estimates of the volume of air that should be withdrawn and introduced every hour, for each person, in public institutions of different kinds: Children's schools, twelve to fifteen cubic metres; schools for adults, twenty-five to thirty cubic metres; amphitheatres, thirty cubic metres; assembly-halls and long-continued meetings, sixty cubic metres; play-houses, forty cubic metres; barracks, thirty cubic metres during the day, forty to fifty cubic metres at night; hospitals for the ordinary sick, sixty to seventy cubic metres; hospitals for the wounded and for women in childbirth, one hundred cubic metres; the same in times of epidemic, one hundred and fifty cubic metres; prisons, fifty cubic metres; stables, one hundred and eighty to two hundred cubic metres. These numbers certainly represent the maximum of reasonable demands; and M. Bouchardat thinks that they are exaggerated and not justified by clinical experience. Besides effecting the renewal of the air, ventilation also furnishes the means of obtaining a nearly constant temperature—in winter by means of the circulation of hot air through the house, in summer by air drawn from the cellar. The latter method is quite effective for securing an agreeable temperature in hot weather without much expense, whenever a sweet, dry cellar can be had. The cabinet of the Conservatoire des Arts et Métiers, in Paris, is kept cool in this way, the draught of air being promoted by gas-jets kept burning in the ventilating shafts; as is also M. Daville's laboratory at the Normal School, where the opening of a few squares in the glass-roof furnishes the required stimulus to the circulation. Similar principles have been adopted at the palace of the Corps Législatif. The subject of applying the artificial refrigeration of the air in colonial life in hot countries has been studied by M. Dessoliers, and elaborated by him with a number of ingenious devices, among which the storing of cold night-air for use during the day plays a part.
In temperate climates the principal object of ventilation is the replacement of vitiated air with fresh. Artificial ventilation is produced either by inducing a movement of air by means of draught-chimneys, or by forcing in air through the agency of mechanical ventilators. A trial has been made at the Lariboisière Hospital of a system of ventilation in which the air is drawn from the roof and forced into flues that ramify into the several halls to be ventilated. At the moment of entering the halls the air is heated by being brought in contact with steam-pipes, so that a uniform temperature of 78° is maintained in the wards, with an atmosphere free from odor. Notwithstanding purity of air is secured, the mortality in this institution is not inferior to that in non-ventilated hospitals. This is attributed by M. Bouchardat to the mischievous influence of the high temperature which they endeavor to maintain. He favors heating and ventilation by open fire-places. This method is preferred in London, where fires are kept up in summer as well as in winter, at least in the principal office of the institution, and the windows are opened at all times when it is possible, while mechanical ventilating apparatus is used only exceptionally. The air, sucked in by the strong draught of the chimneys, enters by the joints of the doors and windows. The patients enjoy the sight of the fire and the pleasant feeling of direct radiation, while they collect around the hearths and breathe an air that has not been changed by contact with a heated surface. Possibly the English go too far in this direction. "The importance of pure air," says M. Proust, "has perhaps been exaggerated in some cases by the English physicians, whose example the Americans have followed. It is advisable, according to them, to leave the larger openings, no matter what the weather may be, the windows of dormitories and bedrooms, open during the night. These principles, almost universally observed in the countries of which we speak, entail, in our opinion, great inconveniences." There is really some danger in exposing one's self to cold during sleep.
The study of the questions of heating and ventilation has made considerable progress in France during the last fifteen or twenty years. The construction of numerous school-houses has especially been the occasion of many praiseworthy improvements, but much still remains to be done. Dr. Larget, in an interesting work on rural habitations, has pointed out an apparent relation between the number of openings indicated in the tax-list of doors and windows and the mortality. The general average, for France, of the number of openings per inhabitant, is one and a half. In one hundred departments, in which the number is less than the mean, fifty-five show a higher mortality, and forty-five a mortality equal to the average; while, in a hundred departments in which the number is greater than the mean, sixty show a lower rate of mortality than the average, and only twenty-five a higher rate.
Another point which is too easily forgotten is that, like the walls, floors are permeable to the air. The atmosphere is not bounded by the level of the soil, but extends below it to a considerable depth. The most compact soils include a considerable volume of air, as well as an ever-varying quantity of moisture. When we pour water into a vessel full of well-packed gravel, and displace the air which is present, we find that it generally forms one third of the total volume of the mass. The porosity of the earth sometimes reaches fifty per cent; and miners and well-diggers accidentally buried under cavings-in have sometimes been known to live for several days by the aid of the air circulating through the earth.
Porous soil does not become impermeable to air till below the level at which the subterranean water ceases to exist. Frozen ground does not lose its porosity by the solidification of the water. Incessant interchanges are taking place between the underground air and the free atmosphere. It is by such means that infiltrations of lighting-gas impregnate the soil of the street, penetrate sewers, and cause ills which are wrongly attributed to typhoid affections; and this is most liable to take place in winter when the rise of gas from the soil is promoted by the draught of the chimneys. Ventilation is thus partly carried on through the floor, to such an extent that the atmosphere of a room sometimes contains from ten to fifteen per cent of air from the ground. Hence the danger from impurities absorbed by the soil. They rise, pitilessly returning from the earth, as if to chastise us for our carelessness. The air included in a garden-soil, and generally in any soil rich in organic matters, always contains a strong proportion of carbonic acid. At the same time the oxygen is in diminished quantity, proving that the carbonic acid proceeds from slow combustions, and not from subterranean emanations. According to the observations of Pettenkofer, Fleck, and Fodor, the proportion of acid increases with the depth, and at a few yards beneath the surface sometimes exceeds ten per cent. This presence of carbonic acid is a sign of the activity of the life in the soil. We do not know the exact manner in which the soil and subsoil intervene in the etiology of endemic diseases and the appearance of epidemics. It is a subject of active controversy. We can, nevertheless, approve the teaching of the hygienists who advise us to render our dwellings independent of the soil-air by making provisions for aeration under the basements, or by making the floors impermeable.
Parks and gardens are beneficial, not only because they give a degree of shade and coolness in hot weather, but also because vegetation absorbs waste matter and purifies the soil, and thus diminishes the liability to epidemics. It is well, for other reasons, to increase these oases in cities where the air is not directly vitiated. But the quantity of oxygen which the plants disengage is too small to be made an object. The phenomena of vegetation are extremely slow of accomplishment. Vast spaces and a long time are needed to produce the grass and the wood that are consumed in a few hours. Oxygen is absorbed more rapidly than it is set free. We shall also have to give up the prevalent idea that a little verdure can improve the atmosphere of a room. The advantage of plants, as Dr. Pettenkofer remarks, is rather in their moral than in their physical influence. Public gardens are also desirable because they enliven the view. Even on hygienic grounds, we should be careful not to underestimate the importance of whatever acts upon the mind. We have endeavored, in this and a former essay, to study clothing and the habitation, with particular reference to their relations with the atmosphere; but, even as thus limited, the subject has proved to be a very complex one, and in our progress we have struck upon more than one question that is still imperfectly elucidated. It may, however, not have been without use to attract attention to these questions, which demand new investigations. Hygienic societies are multiplying; departments of hygiene have been created in numerous cities; and the hygienic conferences which have been held at Paris, Turin, and Geneva, attest the growing interest that attaches to the development of a science all of whose conquests redound to our physical and moral profit. Every facility should be given for widening its scope and extending its sphere of action. Diseases that might have been avoided constitute the heaviest taxes that can be laid upon a city.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.