Popular Science Monthly/Volume 10/April 1877/Relations of the Air to our Clothing
|RELATIONS OF THE AIR TO OUR CLOTHING.|
By Dr. MAX VON PETTENKOFFER,
PROFESSOR OF HYGIENE AT THE UNIVERSITY OF MUNICH.
THE committee of the Albert Society has honored me by an invitation to give a few popular lectures at Dresden, on subjects of public hygiene. Let me state to you at once what I think of popular lectures in general.
What ought they to be, and what can we expect from them? I am not one of those who, in all their work and aim, look out directly for the practical use, for the return on the capital, immediate or prospective; but, on the other hand, I feel myself bound, in a certain degree, to inquire into the object of much that may appear to be either unprofitable or useless.
There is no doubt that popular lectures on scientific subjects will not impart really competent knowledge, and will not form experts. Therefore it will be maintained by many that such lectures produce more evil than good, creating as they do, and augmenting, that dilettanteism from which our period is already suffering. In our schools also this dilettanteism is gaining such dimensions, that one might get thoroughly frightened at the immoderate expansion of young people's knowledge, were it not for its small depth, which lessens the danger, and for the fact that the forgetting keeps pace with the learning. Accept my open avowal, that I also am unable to invalidate the objection that popular lectures on scientific subjects are not able to impart a really competent knowledge, and do not form experts.
But I believe that this does not matter, and that they have no such purpose. They are neither an exhaustive, scientific, nor a practical instruction, but a scientific edification and elevation, which are to raise our minds and hearts, and to affect us like listening to good music—to a symphony, the purpose of which is certainly not to make musicians of all the listeners. It is sufficient to feel the harmony which lies in the nature of good music. There is harmony in all our knowing and doing, our aiming and striving, as far as there is truth in them, and fortunately the sense for perceiving this harmony is as widely spread among mankind as the sense for music. This harmony, which pervades every truth, ought to be brought home to the consciousness and feelings of everybody, so that the greatest number may rejoice and become interested in it, that we may approach new subjects, and perhaps make them our study, or that, at all events, knowledge and resulting sympathy may induce us to lend our help to those men whose profession and calling require them to enter more minutely and exactly into the subjects in question. In this respect popular lectures have a high and serious mission. It is their mission to create correct general ideas, to facilitate our grasp of them, to awaken and spread a certain love for different tasks of mankind and of the period, to form ties of friendship between things, ideas, and men. Sympathy and sacrifices cannot be expected or asked from us, if their objects are unknown to or badly understood by us.
For these reasons it is my desire to awaken your interest for some subjects relating to hygiene, and particularly to impress upon you most vividly how much in this respect remains to be done and created, a work we all ought to take our share in.
One of the incessant wants of man is air.
We want air mainly to nourish us and to keep us cool. The quantity of air inhaled and exhaled by an adult in twenty-four hours amounts on an average to about 360 cubic feet, or 2,000 gallons. What we take in and give out during twenty-four hours, in the shape of solid and liquid food, occupies on an average the space of 52 pints, which is equal to 3000 of the volume of the air passing through our lungs. It will astonish you to hear, perhaps, for the first time that this amounts to 730,000 gallons in one year, and to be reminded of that continuous work, which goes on day and night—a never-ceasing bellows-blowing, by which the organ of our life is kept in play. Of course, the quantity of air flowing round the surface of the human body is much greater than that. Do not object, that air is something so light that it need not be taken into account. It has some weight; water, certainly, is 770 times heavier, but our daily 2,000 gallons have for all that a weight of 25 pounds avoirdupois. Still, as it is not my intention to dwell here upon the subject of our oxygen-alimentation, I will to-day consider only the second use we make of the air, the cooling of our working machine.
You all know that life is bound up with chemical processes, kept in continual activity through the ingestion of solid and liquid food, and of oxygen from the air. One of the conditions for the normal performance of these processes is a definite temperature, above and below which they (although not brought to a standstill) go on differently—they leave off performing the functions of normal life—they lead to disease or death. With man this uniform temperature of his organs is one of the most essential conditions of his life. The blood of the negro living in the torrid zone of the equator is not by one-fifth degree warmer than that of the Esquimaux in the highest north at the coldest time of the year—it is always 992° Fahr. The extremes of temperature under which human life exists are 95 to 104° Fahr. in the tropics, and 57° to 84° under freezing-point in the polar regions. There are even differences of 72° in the mean monthly temperatures of some countries, and yet the organs of man are everywhere of the same temperature.
By what means is man enabled to meet such colossal differences? What are his weapons for sustaining this gigantic struggle?
Let us look a little nearer into the absolute quantities of heat the living organism has to manage. The chemical processes going on in an adult person, within the space of twenty-four hours, produce about 12,000 caloric units. By caloric unit natural philosophy designates that quantity of heat which is necessary to raise the temperature of one pound avoirdupois of water by one degree of Fahrenheit. By the heat produced by one person during one day about 660 gallons of water could be made warmer by nearly two degrees, or 73 gallons could be heated from freezing to boiling point, from 32° to 212° Fahr.
Under certain conditions man produces more or less heat; for instance, according to the quantity of food he takes, or the degree of muscular exertion he undergoes, such deviations from the mean amounting at times to 50 per cent. of the whole quantity; but it is always the task of the body, and a strict condition for the maintenance of health, to keep the heat of the blood substantially the same, or at least within two degrees.
We have to look upon ourselves as warm and humid bodies placed within a cooler atmosphere. Such bodies lose their heat in three different ways: 1. Radiation. 2. Evaporation. 3. Conduction. This triple arrangement is of great advantage for the heat-department of our organism, inasmuch as the existence of these different routes allows of a delicate regulation—that, for instance, which we lose in a given case by radiation can be made up by diminution of loss through the other routes, and vice versa. The losses by radiation and by conduction are the most constant under equal conditions, and evaporation of water is the principal means for equalizing differences resulting from varying production of heat or from difficulties of the two other routes. Allow me to illustrate this by drawing your attention to some every-day phenomena.
You arrive, for instance, in an hotel after a journey during a cold winter's day, and have at once a fire lit in your room. Let the fire be ever so bright, the thermometer even rise to a reassuring degree—you must stick to the fireplace; the room does not get warm. If you continue to live in the same room and have the fire kept in, it will by-and-by get comfortable even if the thermometer in the room should stand lower than on the first day, and you will think quite correctly that the room wanted time to get warmed through and through. Before that had taken place, the loss of heat by increased radiation into the incompletely warmed space made itself sensibly felt in the heating department of your body. Radiation is the stronger the greater the difference of temperature between the two bodies. Surrounded as you are in a room not only by air, say of 68° Fahr., but also by walls, furniture, etc., which stand, perhaps, at 38° to 40°, your body radiates its heat particularly toward these colder objects, till they also get warmer. For a room to be warm, it must get warmed with all which it contains.
Let us now look at the contrary case, when our loss by radiation is uncommonly limited; for instance, in a thronged room on a warm and moist day. You feel an oppressive heat, and scarcely trust the thermometer, which marks only 68°, perhaps your favorite temperature. Quite correctly, you accuse the throng of people, and retire into an adjoining room, where you find the air delicious, and seem to receive new life; there, again, the thermometer is suspected by you, as it is scarcely different from its colleague inside; and if the air in the two rooms were to be examined eudiometrically, the difference would be so small as to leave unexplained the difference in your sensations. What, then, causes this difference? It is the suppression of your lateral radiation of heat, when you are in the midst of other equally warm bodies; your receipts and expenditure by radiation cover each other, and the cooling of the individual limits itself to the two other routes, conduction by the air moving round him, and evaporation of water from his surface. On such occasions the pores of your skin pour forth a quantity of water, and, at the same time, you instinctively try to increase the movement of the air—that means, its quantity in proportion to your surface; you want to increase your loss by conduction, and, if possible, by evaporation, and take to fanning, in order to facilitate the departure of your rising heat by the two open routes.
The loss by radiation can be very considerable under certain circumstances; 50 per cent. of the whole quantity of heat generally going that way, it is obvious that radiation deserves our full consideration. Particularly, an unequal radiation may be very injurious, such as takes place when a person is sitting or lying near a cold wall which is not covered by some bad heat-conductor, or near a window, etc.
On school-forms, the exposed sides of the first and last pupils are always more cooled than the sides directed toward their neighbors. In this respect there are numbers of practical points which are far from being sufficiently taken into consideration.
Let us now consider some instances in which the abstraction of heat by evaporation is predominant, or preeminently felt. The best known is that experiment by which one tries to learn the direction of the wind when the air appears calm and the sky cloudless. The moistened forefinger feels colder on that side which looks toward the wind, because more evaporation takes place there. The experiment does not succeed so well when the air is moist, because the moisture in the air prevents further reception of moisture by it; in our case, preventing the evaporation from the moistened finger.
Our organism acts similarly in all cases where there is an increased production of heat in our body, or where less heat is sent away by the two other routes. It has the power of dilating or narrowing the small blood-vessels in our skin and internal organs. The blood-vessel nerves which govern this motion are not subject to our will, but liable to be excited by external causes. When a person blushes, he gives off heat, because more blood rushes into the dilated blood-vessels of his cheeks and periphery generally, and more heat leaves the body. Under similar circumstances the whole surface of our body becomes fuller of blood and warmer, there is more heat to radiate and to be conducted away, and to be consumed by increased evaporation of the watery part of the blood.
The great value of evaporation for the cooling of our body can be estimated by calculating that as little as fifteen drops of water requires 24 caloric units to be changed into vapor.
We have at Munich a great apparatus for studying the process of respiration. It was given by the late King of Bavaria, Maximilian II., to the hygienic department of the university. Prof. Voit and myself have, by aid of this apparatus, investigated the quantity of water evaporated by men and animals during twenty-four hours. The constant result was that, under other similar circumstances, the quantity of evaporated water always rose in proportion to an increased metamorphosis of tissue, whether this increase was the consequence of increased nutrition, or of muscular exertion. We have experimented upon men at rest and at work, and we have found that on a day of rest they usually evaporated through lungs and skin about two pounds only during twenty-four hours, and on a day of hard work 44 pounds of water. In the first instance, about 2,016 caloric units, in the second, 4,480 had to leave the body in consequence of evaporation.
This explains to you how it can be that even with the hardest work our blood will not become warmer, but sometimes even cooler. The last observation has been made quite recently in mountaineering expeditions. Prof. Lortet, of Lyons, found, when he made an ascent of Mont Blanc, that the temperature in his mouth and armpit was less than normal, and became normal only when he was at rest. On such high mountains the lessened pressure of the atmosphere favors the peripheric circulation, there is a rush of water to the surface, and its evaporation takes place more readily, and increases with the altitude. At great heights persons in a balloon constantly complain of great dryness in the mouth.
Profs. Voit, Recknagel, and myself, are just now occupied in investigating the economy of animal heat, and we have found that after six hours' hard work the person leaves the apparatus in a cooler condition than when he went in, or after he had been at rest in the apparatus for the same space of time. Of course, the ventilation of the apparatus must work well, and send per hour about 11,100 gallons or 1,800 cubic feet of air through the chamber, else less water and less heat depart by evaporation.
You see what powerful means of cooling our body we have in the increase of our peripheric circulation, and consequent evaporation, at a time when the other routes are not open sufficiently but you see also how dangerous this means can become, if it is employed at a time when considerable quantities of heat depart on the other routes. If, heated and damp, you enter suddenly a cold space, where radiation increases at once, and a good deal of heat is also yielded by conduction to the cold air, you are in great danger of contracting an illness by the abnormal losses of heat, and the violent and sudden changes in the circulation. But if you undergo such changes slowly and gently, the three routes open themselves harmoniously. Our organism is a faithful and clever servant, who helps himself and his master, provided he is not hurried and ill-treated. When I come to speak of ventilation, I shall not forget to tell you of currents of air, called draughts.
The third route, that of conduction, by which we give up heat to the air, is also of great importance, and must in some circumstances replace the two others to a considerable degree. As long as our body is warmer than the surrounding air, this air gets warmer at every point of contact with our body, but at the same time lighter, and as such it is displaced by colder and heavier air, which in its turn gets warmer and lighter, and so on.
Each person standing in the still air of a room causes in this way an ascending current of air, just like a heated stove. A very sensitive anemometer, placed between coat and waistcoat, shows the existence of this current, which is strong enough to set the little wings of the instrument in play. The air in this room appears quite still, and yet it is in thousand-fold motion and ceaseless restlessness; but, happily, our nerves are not aware of this, just as a short-sighted person may deny the existence of some object, till his eyes get the assistance of a glass. Whoever of you would be able to feel or see all the movements of the air in this room would probably not be able to stand it. A correct idea may be formed about it by the action of smelling substances. If, for instance, an escape of gas were to take place in a remote corner of this large room, you would become aware of it almost immediately all over the room. Our nerves are happily so organized that they begin to feel the motion of the air only when it amounts to about 34 feet per second.
You may have some doubt about this ignorance of your nerves, because the proof lies not in our immediate perception, but in conclusions from other observations; but you may easily convince yourself that it is so. It is the same thing whether you move your hand at a certain rate through a still air, or whether air moves at the same rate round your hand. You will find that you do not feel anything, no resistance, no coolness, if you move your hand at less than 19 inches per second.
I take this opportunity to draw your attention at once to the average movement of air out-of-doors, a subject very imperfectly known to most people, but which you must understand well in order to have a correct idea of the real difference between being in a room and in the open air. The velocity of the air is measured by an instrument called an anemometer, a description of which you can easily get at. In our temperate climate this velocity amounts on an average to about 10 feet per second. This would make about 7 miles per hour. Imagine a frame about the height and width of a human body; let us say it measures about 6 feet by 12, or 9 square feet. If you multiply this by the average velocity of the air, you will find that in one second 90 cubic feet, in one minute 5,400 cubic feet, in one hour 324,000 cubic feet of air flow over one person in the open. I shall come back again to these numbers when we have to consider the subject of the ventilation of dwellings, but you will already understand that it is not too much if 2,100 cubic feet of new air per hour and per bed are considered necessary in the ventilating arrangements of hospitals, etc. This quantity, which appears large, is after all only about 160 of the quantity of air which comes in contact with a person in the open at the above stated average velocity of the air.
You see, therefore, that we give off more heat by conduction in the open air than in a room, and in the latter proportionately more by radiation and evaporation.
The power of conduction is best appreciated when we change the air for some other fluid medium, which is a better conductor than air, and more capable of absorbing heat, I mean water. In air of a few degrees of heat, we can feel pretty comfortable with moderately warm clothes; but, if with the same amount of clothing we were to get into water of the same temperature, we should feel painfully cold, and should probably be frozen to death in a few hours, although our loss by evaporation would have ceased entirely, and that by radiation nearly so. In hot climates, therefore, a daily bath is of great service for the necessary cooling of our body, even if the water is not cooler than the atmosphere.
In the air also the loss of heat by conduction is the greater the lower the temperature, and the greater the velocity of the air which flows around us. This explains on the one side why it appears superfluous in a calm and cool air to make use of a fan, while this expedient acts so beneficially at higher temperatures; and on the other why, as a rule, a warm air in motion appears much cooler than a calm one of equal temperature. Think of the sultriness before a thunder-storm, as long as the air is at rest, and how differently we feel as soon as the first wind rises. The air is not yet cooler, not less saturated with vapor than before, and still it deprives us of so much more heat that we deem it less sultry, even cool, only because it travels over us faster.
When we fan ourselves in a hot and damp air, the same thing takes place—then, also, a greater amount of air passes over us in a given time than if we leave the air to its own motions. The fan changes nothing in the temperature and moistness of the air, it only increases its velocity, and in consequence the abstraction of heat, and thus affords us coolness chiefly on the uncovered or only slightly covered parts of our bodies; therefore, ladies have more reason for using it than the stronger sex.
As long as the air is our surrounding medium, an increased evaporation associates itself with the increased loss by conduction, at least as long as the circulation of the blood in the skin remains active and the air is not saturated with moisture. The fan scarcely ever cools by increased conduction alone, but also by increased evaporation. Therefore, fanning with dry air is much more cooling than fanning with a moist air of equal temperature. We all know how much quicker wet roads and wet clothes dry when there is a good wind. However rapid the motion of moist air may be, it does not dry. When our body is bathed in perspiration the fuller condition of the skin occasions an increased transfer of heat from the dilated blood-vessels to the surrounding air by conduction, but generally also by evaporation.
In southern climes, at the hottest and moist time of the year, when the body cannot lose much heat by radiation toward colder objects, when the temperature of the air approaches and even surpasses at times that of our blood, the European often feels the heat to suffocation, and besides the use of the bath he has no other practical remedy than the fan and the shade.
In the shade the air is not only cooler, but also more in motion. The difference of temperature between a place sheltered from the rays of the sun and a neighboring one exposed to them, produces a motion, a current, because bodies of air of unequal temperature are also of unequal weight. They are not in equilibrium, and seek to reestablish it by motion. Any one may easily convince himself thereof who, on a hot day with calm air, walks alternately over places exposed to the sun and sheltered from it. As soon as he comes into the shade of a cloud, a house, or a tree, he feels at once a soft wind rising. The shade not only protects us against the direct solar rays, but it increases also the ventilation of the shady place.
The fan acts on the same principle. The pankha in the bungalow, by increased conduction and evaporation, keeps the blood of the European at its normal temperature of 992°. When the temperature of the air rises to 140°, when the walls of the house or bungalow are no longer cool enough to provoke radiation from the heated human body, man is reduced to cooling by evaporation. It greatly depends upon the state of dryness of the air how far he succeeds. The drier the hot air is, the better is it able to withdraw water from the skin, from the respiratory organs, from the wetted floors, and consequently the more heat from the human body. The moister it is the less it is able to act thus.
In order to give you an idea of the quantitative differences in play, we will consider the losses of heat by respiration as they take place at different temperatures and different conditions of moisture of the air we draw in. In twenty-four hours the quantity of this air is on an average 2,000 gallons. It has been calculated that by the process of respiration a person loses 1,1 72 caloric units when the air is at 32° and quite dry, 1,116 when it is half saturated by water, 1,060 when it is completely so. The difference between the two extremes is only a small percentage of the whole loss. But, when the temperature is 86°, the above numbers would be respectively 1,096, 760, and 420.
A comparison of the losses of heat by the respiration of an absolutely dry and an absolutely saturated air at 32° and 86° Fahr. is highly instructive. We lose:
|at 32°||and dry||1,172||caloric units.|
|at 32°||and saturated||1,060|
|difference as much as||640||caloric units.|
The different state of dryness of the air appears thus to be of a greater moment than the difference of temperature, and this is the reason why our sensations do not always coincide with the thermometer. You readily understand how much more difficult it is to manage one's heat-household in a hot than in a cold climate. Our means for warming ourselves are better than those for carrying off our heat. Therefore the European race has a hard fight under the equator. The working power of the body depends on a certain amount of consumption, by which a certain amount of heat is necessarily created, which has to leave the body in a regular way. The Hindoo who has to draw the European's pankha, bears the heat better in proportion as he takes less food and creates less heat in himself, but then his working-power is also quite proportionate to the total of his consumption.
The European's struggle in a hot climate and his dangers of degeneracy will remain the same as long as he has no better means of cooling himself by some or all of the known three routes. Houses with thick stone-walls are tolerably efficacious. These walls rarely get warmer than the average temperature of the year. They cool the air which comes into the house, and act on the inmates in the way we have seen when speaking of the room which is not warmed through. A good means would be some contrivance by which the air in the house could be deprived of its water.
I could not help inflicting upon you this rather long introduction, nor could I possibly abbreviate it, as, without the little knowledge which I have tried to impart to you about the cooling of the human body, you would not be enabled to obtain a proper insight into the functions of our clothing and our dwellings. Therefore I believe myself to have had a good claim on your patience and indulgence.
One of man's principal defensive weapons in his struggle for existence is his clothing. The place it takes in the history of civilization and its connection with physiology are not often thought of. People speak about it generally from a moral and aesthetic point of view, but the main purpose of clothing is seldom approached in conversation—I mean the purely hygienic one. I deem this to be a misfortune, because this forgetting of the chief point has subjected mankind to the rule of small and frivolous considerations, and the manners and fashions of the period get frequently the better of the hygienic fitness of the clothing. Morality and beauty do not depend on dress. They cannot be created or preserved by it. These great qualities could even exist without it, but the human body as it is could not, or only barely and imperfectly, exist in our climate without the protection of clothing, which is more indispensable for our health than for our beauty and morality.
So manifold are the changes brought about in our system by clothing ourselves, that I am unable to give you more than some incomplete parts of the subject. When I cover one part of my body I change the degree of abstraction of heat by all three routes known to you, but without obstructing any one of them entirely.
To speak in the first instance of radiation, it will be clear to you that our surface is prevented from radiating heat directly toward the colder objects in our neighborhood, and that it can only radiate toward the covering materials, which receive this heat. By the laws of conduction and radiation the heat, which has radiated from the body into the clothes, has to travel through them by radiation and conduction, till, arrived at their outer surface, it can radiate thence toward colder objects, just as it would from the naked surface of the body. Thus by our clothes we keep the heat radiating from us somewhat longer in the immediate neighborhood of our surface. The lightest covering even makes itself perceptible by impeding radiation, the thinnest veil keeps warm in some degree. It is just the same with the earth itself. On a calm, clear night the earth's surface becomes so chilled by radiation into the colder space, that the moisture of the air precipitates itself on it as dew, and at times as hoar-frost, and even as ice, just as the moisture in a warm room does on a windowpane cooled from the outside; but, when a veil of clouds overhangs the earth during the night, the earth never cools itself so much as to allow of any dew forming.
There are substances, called diathermal, which allow the rays of heat to go straight through them without any absorption, for instance, the crystals of common salt, but all the materials of our clothes are such as absorb the rays of heat which come to them from one side, and only part with them after they have reached the outer surface. The transit of heat through what we may call our artificial surface depends essentially on the conductive power of the material and its thickness, i. e., on the length of time and way which the heat has to go through in order to travel from our surface to the outer surface of the garment.
Thus the whole immediate neighborhood of our body is continually warmed in an even degree by our radiating heat, and our sensitive skin is spared the numerous disagreeable or injurious effects of a rapidly-changing temperature.
The heat does not remain in our clothes, it is continually on an outward move, faster or slower, and, to a certain degree, also warms the stratum of air between our clothing and our skin, so rich in nerves and blood-vessels. This air, as we shall see presently, continually changes, and must change if we are to feel comfortable. In the cold of winter, and in the open air, we lose our bodily heat out of our well-selected garments without any sensation of cold, only because we have removed the place of exchange between the temperature of our warm blood and the cold winter air from our sensitive surface to a substance without life and sensation; instead of our skin, our dress feels the cold. It is the same with the hair of animals, and the feathers of birds, they are also without nerves.
Id proportion as our heat-losses increase, while the creation of heat in our interior remains about the same, we feel the necessity of diminishing the rate at which the heat leaves our immediate neighborhood. This kind of regulation is somewhat taken care of involuntarily even by the naked body. In consequence of the cold, the nerves which act on the calibre of the blood-vessels of our surface contract them, and lessen the quantity of blood in them, so that less heat comes to the surface, and we need not be afraid of becoming also inwardly colder if we feel cold, even very cold.
The sensation of cold on the skin does not necessarily give the measure of our internal temperature. In the cold stage of ague, for instance, the temperature of the internal organs rises considerably, while by a kind of spasmodic contraction of the superficial blood-vessels the flow of heat toward the skin is less than normal. The above-mentioned regulation of heat-loss by the capillary system of our skin is not all-sufficient either in point of time or degree. The cold may be too strong, and the regulator get overworked and paralyzed, so that additional clothing is required to delay the departure of our heat, and to spare the nerves of the blood-vessels. We help ourselves by additional clothing, and the underlying article of clothing stands in the same relation to the outer one as the skin to its first covering. From this point of view you have to consider the sequence of shirt, under-clothing, coat, overcoat, etc., etc., an arrangement by which we save the vasomotor nerves the greater part of their work.
It is an open question, which the incompleteness of our hygienic knowledge prevents us from answering quite satisfactorily, how far we ought to hand over the regulation of our heat-loss to our dress, or how far we should go in deputing it to our organism, and its capability of transferring more or less heat from the centres to the surface of our bodies. This self-help of the organism and the readiness for it resulting from frequent exercise of this function are generally called hardening one's self; the contrary, making one's self tender. The former we can never quite dispense with, but I believe that too high a value is sometimes put and too large claims made on it. One ought to possess the capability and the readiness, but not to make use of them continually.
All human aim must be to obtain the greatest effect at the smallest expense. We ought to choose those means which attain the end without exhausting our power, which should be preserved for higher purposes. These principles ought to guide us in approaching the question. It is not only superfluous, but positively injurious, to use one's self up.
I believe that it is now evident to you that a part of the heat of our body radiates from the surface of our clothing; but we must now consider whether this radiation does not vary according to the nature, quality, or color, of the material. Experiments which have been made by Dr. Krieger on wool, wash-leather, silk, cotton, linen, and India-rubber, have not shown any important difference. Krieger covered cylinders made of tin and filled with warm water with different and differently-arranged materials, and noted the decrease of temperature in stated periods. He used layers of two different materials, but it made no great difference what the outer layer was. Still, I will mention that silk and cotton allowed more heat to radiate than wool. The color also of the material has been shown to have no great influence on the radiation of heat, which remains the same, whether we have a black or a white garment on.
But it is quite another case when we receive luminous heat, rays of heat proceeding from luminous bodies, such as the sun, or some flame; then differences result, which certainly are not very great with different materials of the same color, but become great indeed when the colors are different. For white textures the following proportions are found:
|When cotton received||100|
With shirtings of different colors the proportions were:
Of course, you all know by experience that, when dressed in black, you feel much hotter in the sun than when dressed in white. It is remarkable that, pale straw excepted, each color heightens considerably the absorption of luminous heat-rays, and that blue does so nearly as much as black. But, as soon as we are in the shade, the differences nearly vanish.
If we continue to consider our loss of heat by radiation through and from our clothing (omitting for the present conduction and evaporation), we come at once to the practical question, how much this loss is retarded by interposing several strata of material between our surface and the air, or in fact to the question about the heat-conducting power of materials and textures. Very few experiments have been made in this respect. We know, with respect to this point, the properties of metals, of minerals, of chemical compounds, but not of wool, linen, or leather. This shows, by-the-by, how little hygiene has been treated until now in an exact and scientific way. We talk in a general way about the use of garments as bad conductors of heat, but the few experiments known to me entirely run counter to our accepted ideas.
Krieger experimented on cylinders filled with warm water, by surrounding them tightly with single or double textures. As the loss by radiation is the same in both cases, any difference must result from difference of conductive power in the coverings of the cylinders; but the results were, for the most part, surprisingly small. The following numbers represent the proportions of loss of heat through double tight-fitting coverings in comparison to single ones; the losses through the single ones being taken as 100, they were, through—
|Double thin silk||97|
|"Thick home-spun linen||91|
The whole question is certainly not exhausted by these experiments, but one thing becomes evident by them, that it is not the substance and its weight, but the texture and the volume, which are the principal causes of the difference. Thin and stout silks, fine and stout linen, are nearly equal in substance, and equal sizes of them are not so very different in weight; it is their different heat-conducting power which causes the difference of the loss, and this is, even through two layers of them, not as much as ten per cent, smaller than through a single one.
By other experiments one can demonstrate that, by changing the shape and volume of the same substance, great changes of heat-loss can be produced, although the substance and its weight remain the same. If you cover the tin cylinder, previously filled with warm water, with common wadding, and observe the falling of the immersed thermometer, you will be astonished to see how rapidly the fall goes on, as soon as you compress the wadding firmly and diminish its volume: the outward flow of the heat increases by forty per cent.. The same, you know, is the case, when a wadded garment is worn out; the quantity of the wadding is the same, but its volume and its elasticity have undergone a change, and you will find it considerably less protecting.
This observation leads to another instructive experiment, relating to the influence of double layers of material. If the first layer only is tightly drawn over the warm cylinder, and a free space of one-third to one-half an inch between it and the second, which may be compared to a comfortably-fitting garment, the second layer very considerably lessens the outward flow of the heat. The amount due to conduction being deducted, the impediment by the second layer is about the same for different materials, but very considerable for each of them:
From this follows the practical truth, that we can produce a very different effect on our body by the same number of clothes, according to the tightness and looseness in the make. Just call to mind tight shoes and gloves in winter-time!
This fact leads to a series of other facts, which contain the explanation why wadding, as long as it is loose and elastic, keeps you warmer than when it is once flattened. This is the air contained within the clothes.
One generally considers clothing as an apparatus for keeping the air from us. This conception is utterly erroneous; quite the reverse, we can bear no garments which do not allow of a continual ventilation of our surface. Just those textures which are most permeable to the air keep us warmest. I have examined different materials for their permeability to air, which can be easily ascertained. One closes a series of perfectly equal glass tubes with different textures, and observes how much air passes through the clothing substances at the same pressure during the same time. Taking the quantity of air passing through flannel as 100—
|Chamois||"||51||parts of air|
to pass through them.
If our clothing kept us warm in proportion to its power of excluding the air from our body, kid would keep us a hundred times, and chamois warmer by one-half than flannel, and so on, while every one knows by experience that it is quite the reverse.
If there are several layers of the same material, ventilation loses but very little at the second layer, because the velocity of the air in its passage through the first layer remains about the same on its further progress, the following layers being like a continuation of the preceding ones, as if they were tubes of the same calibre, retarding the original velocity of a fluid by the amount only of unavoidable friction.
Thus, a current of air travels incessantly through our clothing. Its force, as in ventilation generally, depends on the size of the openings, the difference between the outside and inside temperature, and the velocity of the surrounding air. We need not be anxious to make our clothes prevent the access of air to our skin; they have only to regulate and moderate it to such a degree that our nerves may not feel the air as something in motion. This degree is far from immobility. When in the open air we believe it to be quite calm, there is still a velocity in it of at least one foot and a half per second, or about one mile per hour, as you heard before.
Our clothing not only renders the air still around us, but it also regulates its temperature by the heat which leaves our body; we heat our garments, and they continually heat the air passing through the meshes and pores of the texture. We may compare our clothing to a calorifer or stove, warmed by the heat emanating from our body's engine for the purpose of warming the air round our surface.
We do not feel the loss of heat which our clothing undergoes as we should if the air were to strike our surface without having been previously prepared by our dress; the differences of temperature balance themselves within the material we are clothed in, and of which the ends of our cutaneous nerves form no part. Inside our dress we carry the air of the South wherever we may be. Its temperature averages about 75° to 94° Fahr. We live in our dress like an unclothed tribe in a paradisian country, where the air is constantly calm and the temperature 75° to 94°. It will be easily understood now why rough, loose textures keep us so warm, while newly-carded cotton-wool does so more than when old and compressed; why tissues of fine fibres and threads make the best material. Fur, of which you know so well the properties, consists of hair and skin. Chemically speaking, there is not much difference between skin and hair. In fur the weight or body of the skin is much greater than that of the hair, and still it is essentially the light hair to which the fur owes its warming properties.
There are some interesting experiments on this point. Krieger observed the flow of heat after covering his cylinders with unshorn and shorn fur. Putting down the loss of heat through the entire fur as 100, he found that it rose to 190 when the same piece of fur was used shorn. A dried skin, you know, is always somewhat porous. When he altered this by giving it a coat of linseed-oil varnish, the loss of heat rose to 258; and, when he took a solution of gum-arabic instead, it rose even to 296.
It has been proved that the living organism, in parting with its heat by radiation and conduction, behaves just like a tin cylinder filled with warm water. It is a yet older observation that furred animals, such as dogs, rabbits, etc., cannot live when they are shorn and their skin varnished or oiled. One used to explain their death by the suppression of the evaporation from their skin, but it can be proved that even in a comfortably-warm room these animals literally freeze to death. Krieger sheared a rabbit, after having noted its temperature and frequency of respiration; they were 102° and 100 per minute. He did not use any varnish, to avoid any possible suppression of evaporation from the skin, but enveloped the shorn animal in a wet cloth. The temperature of the room being at 66°, the animal lost so much heat that, after five hours, its interior temperature had fallen to 75°, and its respiration to 50 per minute.
A fur is so arranged that its fine hair, projecting into the air, intercepts all the heat, which flows from the surface by radiation and conduction, and distributes this heat through the air, which circulates between the single hair-cylinders; the finer the hair of the fur, the more of the outgoing heat is taken up by the air, which, however cold the temperature may be, reaches the nerves of the skin as a warmed air. Furred animals, in winter, when touched superficially, give a very cold sensation; it is only near the skin that their hair feels warm. In severe cold, certainly little of our animal heat comes as far as the points of the hair, from which it would radiate or be conducted into the air; the current of air in the fur cools the hair from its point toward its roots, and a severer cold penetrates only a little farther into the fur, without necessarily reaching the skin of the same. This takes place only when the temperature is uncommonly low, and the air in violent motion. Travelers in high latitudes all agree that extreme degrees of cold can be borne very well when the air is calm, but scarcely so when there is a brisk wind.
This tends to show that in very severe cold the outflow of heat, by the skin into the air contained in the fur or within the dress, takes place through one route only—that of conduction; when a fur is worn, no heat comes to the surface for radiation, as soon as the points of the hair have the temperature of the surrounding air. Evaporation also sinks to a minimum, because at 68° Fahr. under freezing-point all formation of aqueous vapor already ceases, and nearly all the heat in the fur and the dress is employed to heat the arriving air, whose velocity increases according to the difference of temperature. In a well-furred animal the changes of temperature in the surrounding air only change, if I may say so, the latitudes of the cold and warm zones in the fur; the place where the temperature of the body and the air equalize each other moves between the roots and points of the hair, and for this reason such a well-furred animal is not warmer in summer than in winter. Its blood keeps always at the same temperature, because in summer a great part of its heat leaves at the points only of the hair by radiation and conduction, while in winter the heat departs already near the roots of the hair.
Air-proof fabrics ought to have only a very limited use. In India-rubber or gutta-percha textures we feel highly uncomfortable when we have to undergo much exercise, or have to give off more heat than usual. They become inconvenient, not because they stop the change of air entirely—which they cannot do in fact, on account of the necessary openings in them—but only because they limit the universal exchange of air in the underlying garments. For protection against the wet from without they are well suited, but they produce another wet on our skin by impeding evaporation. They may be used in wet weather, when accompanied with cold or wind, but never, though wet, when it is warm or calm.
Finally, I have to draw your attention to the relations which the materials of our clothes have to water, by which their functions are considerably altered. They are all hygroscopic; that means that they condense from the atmosphere a certain amount of water. This hydroscopic property, very different in different bodies, increases with the decrease in the temperature of the air, so that all of them condense more water at freezing-point than at higher temperatures. Partly, also, the relative, moisture of the air is of some influence, so that at 68° the hygroscopic body absorbs more water from an air nearly saturated than from a less moist air. As yet we do not know much about our clothing materials in this respect. I have made some preliminary researches, and have found unexpectedly great differences.
I took two equal pieces of flannel and of linen, as representatives of the two most important fabrics made of vegetable and animal fibres, and dried them at 212°, a temperature at which they lose all their hygroscopic water. I put them into well-closed boxes of known weight, and noted the weight of the two together. They were then exposed to the air in places of different temperature, and from time to time put back into the tin boxes, and the weights taken again. By this method it was not difficult to ascertain the relative quantities of hygroscopic water which the flannel and the linen had absorbed. These quantities are tabulated below, as they resulted from different localities, temperatures, and lengths of time, the weight of the linen and flannel being 1,000 grammes each:
What most strikes one is the invariably greater hygroscopic power of wool than of linen; the maxima and minima of flannel and linen being respectively 175 and 111, 75 and 41.
Observations 5 to 8 show that linen changes the quantity of its hygroscopic water at a proportionately quicker rate than flannel. The two pieces were for twelve hours in the cellar, when linen absorbed 111, flannel 175; immediately after, for four hours, in a cold place, where linen lost 18 per 1,000 of its absolutely smaller amount of water, while the flannel lost only 15 per 1,000; but during the next three hours linen lost only 2, but flannel 12 per 1,000.
When (Obss. 9 to 15) the pieces had come from the cold lecture-room into a warmed room, linen again ceased giving off water at a much quicker rate than flannel.
The accelerated rate, only in an opposite direction, took place again (Obss. 15 to 18) when the temperature in the room sunk from 65° to 59°.
All bodies become more hygroscopic with a sinking temperature, but the absorption of water and increase of weight, as well as the contrary process, take place proportionately quicker with linen than with flannel.
The more the air in any material is displaced by water, the less it keeps us warm, the quicker it conducts the heat; hence the frequent injury resulting from wet clothes, and the striking discomfort produced by a damp cold. You all know how comfortable we can feel in a walk, when the air is cold and dry, and how differently we feel when it is damp, although not colder. Then our clothes also get much damper, and conduct more of our heat away.
This is not to be underrated. We have seen in the table that 1,000 parts of flannel took up in the cellar 157 parts of water. Take the weight of a whole woolen clothing as ten pounds, and you see that it may absorb one and a half pound of hygroscopic water, which requires about 1,680 caloric units from our body to be evaporated.
Linen and flannel bear the same relation toward water they are wetted with as toward their hygroscopic water. Linen is quickly wetted and soaked, wool more slowly, but linen cannot take up the same quantity. Spilled water has certainly taught you this many times, when you wanted to take it up. It is the same in evaporation, which is also much quicker from linen. Two equal pieces of linen and flannel, weighing each 1,000 grammes, put into water and wrung out till they no longer yield a drop of water, keep back respectively 740 and 913 per 1,000.
But a much greater difference exists in the intensity of evaporation from wet linen and from wet flannel, during equal periods, in a heated room.
|OBSERVATIONS.||TEMPERATURE.||MINUTES.||WATER TO 1,000 GRAMMES OF|
|1||70° Fahr.||· ·||740||913|
It is easy to see from this table how much quicker linen works than wool in every direction.
During the first 75 minutes there evaporated from 1,000 parts of linen 511, from 1,000 parts of flannel 456 water; afterward the reverse took place: in the following 30 minutes 130 evaporated from linen, 148 from flannel, and in the last 30 minutes only 44 per 1,000 from linen, but 115 from flannel.
It is also evident how much more evenly the drying proceeds in wool: in the first 15 of the whole 135 minutes 219 evaporated from linen, in the last 15 minutes 28 per 1,000, while with wool it was respectively 212 and 97 per 1,000. I must not forget to mention that all these experiments were made with pieces of nearly equal size and shape.
It is self-evident that all textures lose their permeability to the air in proportion to their state of humidity, the water partly at least obstructing the pores. Coarser stuffs with larger pores will keep their permeability longer; if the pores are equal, the difference in the adhesion of the water to the substances will come into play. Linen, cotton, and silk are very different in this respect from sheep's-wool. The former become very quickly air-tight by wetting, the latter scarcely so, or only after a longer soaking. Soldiers can tell how damp and vaporous the air becomes under a wet tent, and how quickly the tent becomes airy when it begins to dry.
As the porosity of all fabrics depends chiefly on the elasticity of the fibres of their material, it must be of great importance how far that elasticity keeps under wet and dry. There, again, wool stands apart; its fibres do not lose much elasticity when they get wet: it is not so with other fibres. Wet linen and silk are just like Krieger's shorn fur, when it was coated with varnish or gum-arabic. The greater facility of catching cold in wet linen or silk than in wet wool is in exact proportion to the greater facility with which water expels the air contained in their fibres. Many of you may have learned a lesson from a wet linen or cotton and a wet woolen sock.
On the other hand, there is an advantage in these materials if we want to keep ourselves cool and dry. By means of them we part with heat and moisture from our surface much quicker, and hand them over to other layers for further removal.
To be quite methodical I ought now to treat of the different parts of our clothing and of the fitness of different materials for special purposes. But, to say the truth, Science has not yet done much in this direction.
There is still one of our garments to be considered which generally is not regarded as such. I mean the bed—that piece of clothing in which we spend such a great part of our time. It is equally indispensable to the sick and to the healthy, and at all times it was considered as a sign of bitterest want if a man had no place to lay his head.
The bed is not only a place of rest, it is especially our sleeping-garment, and has often to make up for privations endured during the day and the day's work, and to give us strength for to-morrow. You know all the different substances and materials used for it. They are the same as our garments are made from. Like them, the bed must be airy and warm at the same time. We warm the bed by our body just as we warm our clothes, and the bed warms the air which is continually flowing through it from below upward. The regulating strata must be more powerful in their action than in our day clothes, because during rest and sleep the metamorphosis of our tissues and resulting heat become less, and because in an horizontal position we lose more heat by an ascending current of air than in a vertical position, where the warm ascending current is in more complete and longer contact with our upright body.
The warmth of the bed sustains the circulation in our surface to a certain degree for the benefit of our internal organs at a time when our production of heat is at the lowest ebb. Hence the importance of the bed for our heat and blood economy. Several days without rest in a bed not only make us sensible of a deficiency in the recruiting of our strength, but very often produce quite noticeable perturbations in our bodily economy which the bed would have protected us from.
I wish, therefore, to impress upon you that your charitable exertions for the poor may become extended to the bed, that kind of garment which can make up to a great degree for other lamentable deficiencies, as in food, dwellings, clothing, toward which you are in the habit of directing your efforts.
I am quite aware that I have anything but exhausted the subject of the functions of our clothes, but still I believe that I have directed your attention to such essential points as to convince you of the importance which a scientific consideration of the subject possesses in the interest of the heat-economy of the human body.
As our health is so intimately connected with this economy, a better insight into the laws and proceedings of the same must in the end turn out profitable to health in general.
Thus we have learned in our last glorious war how important it is to provide well for the soldiers' clothing, and that a few days' want of provisions is less injurious to the health of the soldier than perturbations of heat-economy through want of suitable pieces of clothing. Our clothes are weapons with which civilized man fights against the atmosphere as far as it is inimical, the means by which he subjugates this his element. It lies in our nature, in our instinct, in our self-respect, to have good clothes, which ought to be also pleasing to the eye; but we ought to become more conscious of their purpose. Ornament must be the minor consideration, and the tailor ought not to hold his scissors as a sceptre over the hygienic purposes of all dress.
Our period strives after novelty in all directions, also after new forms and styles in dress, architecture, and so on; but nothing new will be created with our old points of view remaining. New points of view can only be gained by new and increased insight into the functions of the dress and the house. This function must determine the form, and will not be ascertained without theoretical study. It was not till we had mastered the theory of the overshot and undershot water-wheel that the turbine could be invented.
The influence of theory on practical development is much greater than is usually supposed and conceded. The discovery and settlement of the laws of mechanics had to precede their application to engines, railways, steamboats, and so on. There would be no difficulty in showing why the great inventions of Watt and Stephenson were not made at an earlier period, and that they were the fruit of seeds which were buried in the theoretical investigations of Copernicus, Kepler, and Newton.
Perhaps our future means for keeping our heat-household will be as different in style and appearance from our present ones as a turbine from an old mill-wheel, or a steam-engine from a horse-wheel.
- Abridged and translated by Augustus Hess, M. D., member of the Royal College of Physicians, London, etc.
- Rankine's caloric units are used by the translator.