Popular Science Monthly/Volume 23/October 1883/Clothing and the Atmosphere
| CLOTHING AND THE ATMOSPHERE. |
By M. R. RADAU.
CLOTHING is a kind of armor to help us in the battle against the elements, the importance of which increases with the rigor of the climate which man inhabits. The house may be regarded as an amplified clothing, to be used less constantly, but as more enduring than other clothing, and capable besides of furnishing a full shelter. Both clothing and the house have been invented to protect us; but a very common error, which has given rise to many mistakes, has been to regard the house and the clothing as designed essentially to isolate us from the external air. The truth is, that they are simply regulators of our indispensable and constant relations with the ambient atmosphere. These relations can not well be comprehended unless we take account of the complex phenomena by which the temperature of the body is kept up in the midst of the most diverse influences. We know that animal heat is produced by chemical changes that are accomplished in the tissues, and principally, but not exclusively, by the combustion of the food which is assimilated and brought into the circulation, where the inspired oxygen transforms it into alcohol and carbonic acid. This combustion raises the temperature of the blood, and the warm liquid, which penetrates everywhere, warms the organism almost in the same way that a house is heated by hot-water pipes. The activity of respiration and the consumption of oxygen are diminished during sleep, but increased in taking exercise, when a part of the heat produced is transformed into mechanical work; but, from birth to death, man continues, without ever wholly resting, to draw the breath that keeps up the fire of life.
Notwithstanding this incessant production of heat, which may be increased or diminished, according to circumstances, by as much as fifty per cent, the temperature of the body continues almost invariable. In health it is always about 98°, and seldom varies as much as 2°; and yet we know that in some regions of the globe the monthly means of external temperature present variations rising to more than 115°, with much wider divergences in extreme cases. In parts of Siberia the extremes range from 70° or 80° below zero to 80° or 90° above; temperatures of from 120° to 130° have been remarked in hot regions in Australia and Asia; and men have been able to support much higher temperatures than these for a short time—Blagden 259° for seven minutes, and a certain Martinez, by wrapping his head in cloth, 338° for a quarter of an hour. Under such excessive heats, the temperature of the blood may rise a few degrees higher than its ordinary extreme; but such cases are abnormal.
Constancy of bodily temperature is an indispensable condition of health to warm-blooded animals. By what means does Nature supply deficiencies of internal heat and eliminate an injurious excess, and, in either case, restore the organs to the temperature which is most agreeable to the regular performance of the functions of life? The means are various. When food becomes insufficient, calorification is effected at the expense of the tissues of the animal, and it grows lean. When heat is produced in excess, the organism rids itself of it speedily by several outlets. The body may be cooled by radiation, by evaporation, or by conduction or convection. It is estimated that radiation generally carries off half, and the other two ways a quarter each, of the surplus heat. These ratios are, however, far from being constant; they vary with external circumstances. Evaporation is the valve that regulates the loss of heat, by completing, at a given point, the action of conduction and radiation.
The intensity of radiation, by which heat is dissipated from the body around, is proportional to the difference between the normal temperature of the body and that of the surrounding medium, and increases in the neighborhood of a very cold body. We may in this way explain the chilly sensation we feel and which persists in a room that has not been used for a long time, after the fire has been kindled, and even after the air in the room has become quite warm; while, after the room has been well warmed up, we may feel quite comfortable in it, even with the air at a lower temperature than that in which we were previously chilly. In the former case the walls and the furniture were still cold and abstracted so much caloric as to provoke radiation from the body. The loss of heat becomes less and the sensation of cold disappears as soon as the objects around have become tolerably warm. This also explains why it is dangerous in winter to stay long near a wall or a window where one side of the body is exposed to be cooled by excessive radiation.
For a similar reason we feel too hot in a room full of people, even when the air is only moderately warm. The presence of a considerable number of persons prevents radiation, and the excess of heat can be carried off only by currents of air, or by a more abundant transpiration. We fan ourselves to expedite the cooling by convection and evaporation, by bringing more air in contact with the skin; and if we leave the room when we are nearly smothered, to go out "to take a breath" in an empty room, we shall be astonished to find by the thermometer that the temperature of the two rooms is nearly the same; only that radiation is free in the empty one. The agreeable refreshment the shadow of the woods gives us is due to the relatively low temperature produced in the trees by their faculty of evaporation, and the facility it affords for promoting radiation from the skin. The body is also cooled by convection, or by giving off its heat to the air that bathes it, and this loss is more sensible in proportion as the air is cooler and more frequently renewed. The atmosphere is always in motion, even when apparently most calm; and thousands of its movements escape our senses, because they are not strong enough to impress our organs. These ceaseless motions, it must be clear, contribute greatly to the cooling of our bodies; but the effect is most marked in the open air, when we are exposed to the action of the winds. In our climate, the average velocity of the atmospheric currents is about ten feet a second, or seven miles an hour. Supposing that the extent of the surface of the body exposed to the currents is one square metre, there pass over a man walking out for an hour about eleven thousand cubic metres of fresh air. In hot climates we seek the shade, not only because the air under it is fresher, but also because it has more motion, in consequence of the differences in density arising from the unequal heating. Notwithstanding all the devices that have been contrived for the reduction of temperatures, it is evident that civilization is possessed of more varied and efficacious means of contending against the cold than of mitigating the effects of the heat. It is for this reason that the European finds it so difficult to acclimate himself under the tropics. The Hindoo reduces his internal calorification by eating little; but he is at the same time defective in energy, and has extremely little capacity to work. Assiduous labor exacts a large quantity of food, while an excess of surplus heat simultaneously results from it; for the organism can convert into mechanical labor only about twenty-five per cent of the increase of heat which it produces under a sustained effort. The problem we have to solve is not to seek for a way of producing less heat, but to find a means of getting rid of that which we do produce.
Water is a much more effective refrigerant than air, because of its much greater conductibility; at the same temperature, a bath of water will refresh one more than a bath of air; but baths are necessarily of limited use. The important matter should be to diminish the temperature of the air that comes in contact with the body.
We have next to consider the effect of evaporation through the lungs and the skin. When the thermometer indicates more than 98° in the shade, the body can no longer be cooled by contact or by radiation, and only a single way is left by which the surplus heat can be dissipated. It can only expend itself in vaporizing the water which transpiration carries to the skin and to the mucous membrane of the respiratory apparatus. The lungs, as a rule, exhale about half as much water as is excreted by the skin. Both together remove about a kilogramme of water every twenty-four hours, disposing of as much heat as would boil five quarts of water; but the quantity of water and of heat removed in this way may be doubled and even tripled when all the channels of transpiration are fully opened under the pressure of an excess of internal heat. The vapor disengaged by these operations is absorbed by the surrounding air with a facility proportioned to the dryness of the atmosphere, or to the degree in which it is removed from the point of saturation. There is a limit at every degree of temperature to the proportion of vapor which the air can contain; and the interval between the points of dryness and of saturation increases with the temperature. An atmosphere at the same time very moist and very hot seems heavy to us because it hinders the evaporation of the water that transpiration brings to the surface of the body. This is why hot and moist climates are so much more unhealthy than hot and dry ones.
When the internal calorification is increased in consequence of violent exercise, the excess of sensible heat is eliminated by a more intense radiation, by ascending air-currents, and by a more abundant transpiration; it thus happens that after several hours of sustained effort we sometimes observe a slight cooling of the body, an effect which is the result of a too rapid using up of disposable materials. Hence, to cite the illustrations given of this fact by M. Bouchardat, dogs, which have run long at the hunt, and the overworked and exhausted children in the Belgian coal-mines, returning to the lodge or to their home, first of all things, before even satisfying their hunger, stretch themselves before the bright fire for warmth.
Thus the means of refrigeration at the disposal of Nature are quite varied; they complement and replace each other according to circumstances. But it is necessary to avoid the too abrupt changes which would surprise the system while it is in the process of accommodation. "The organization," says Dr. Pettenkofer, "is a prudent and faithful servant, which will deliver itself and its master from trouble if it is given time to set itself right and is protected against rude treatment." The body, even when exposed stark naked to the air, is not wholly without defense against heat and cold. It can, up to a certain point, itself regulate the expenditure of caloric by the intervention of the vaso-motor nerves that go to the capillaries of the skin. Cold provokes a shrinking of the little vessels, and, restraining the peripheric circulation, diminishes the radiation and the transpiration to such a degree as to protect the internal organs for a considerable time. Heat, on the other hand, dilates the vessels so that the blood flows to the surface and the caloric is in a certain way driven out. Unfortunately, this automatic regulator, the play of which is commanded by the nerves, is too easily disordered and its springs are too easily relaxed. We can doubtless fortify it by exercise, harden ourselves, and habituate the body to support inclement conditions; and there are peoples and persons who have done wonders in this direction; but the hardening process works under limitations, and its results are not within everybody's reach. The real regulators of the heat of the body are clothes.
The thinnest veil is a vestment in the sense that it moderates the loss of heat which radiation causes the naked body to experience. In the same way a cloudy sky protects the earth against too great cooling in spring nights. In covering ourselves with multiple envelopes of which we augment the protecting thickness according to the rigor of the seasons, we retard the radiation from the body by causing it to pass through a series of stages, or by providing relays. The linen, the ordinary dress, and the cloak constitute for us so many artificial epidermises. The heat that leaves the skin goes to warm these superposed envelopes; it passes through them the more slowly in proportion as they are poorer conductors; reaching the surface, it escapes, but without making us feel the chills which direct contact with the atmosphere occasions, for our clothes catch the cold for us. The hairs and the feathers of animals perform the same function as toward their skin, serving to remove the seat of calorific exchange away from the body. The protection we owe to our clothes is made more effectual by their always being wadded with a stratum of warm air. Each one of us thus has his own atmosphere, which goes with him everywhere, and is renewed without being cooled. The animal also finds under its fur an additional protection in the bed of air that fills the spaces between the hairs; and it is on account of the air they inclose that porous substances, furs, and feathers keep warm.
Experiments to determine the degree of facility with which different substances used for clothing allow heat to escape were made by Count Rumford, Senebier, Boeckmann, James Starck, and M. Coulier. The results were not in all cases consistent with each other, but they indicate that the property is dependent on the texture of the substance rather than on the kind of material, or—as concerns non-luminous heat its color.
The most recent experiments are those of Dr. Krieger, some results of which are cited by Dr. Pettenkofer. He observed the rate of cooling of a sheet-iron cylinder filled with hot water and covered by turns with different cloths. Wrapping it with successive envelopes of wool, buckskin, silk, cotton, and linen, and observing regularly the diminution of temperature in a given time, he found the differences insignificant, not exceeding one or two per cent. The color of the materials did not cause the results to vary any more. It appears, then, that in a dark heat the emissive power and the absorbing power, which is correlative with it, vary but little between one kind of cloth and another. The case is different when we have to do with luminous heat, or the solar rays. With envelopes of linen, cotton, flannel, and silk, M. Krieger observed that the absorption of solar heat increased in the proportions indicated by the numbers 90, 100, 102, and 108. The influence of color was much greater: with cotton goods of different hues he found the numbers to be white, 100; straw-color, 102; yellow, 140; bright green, 155; dark green, 168; Turkey red, 165; bright blue, 198; black, 208. These facts explain why in the hot sun a black coat is warmer than a white one, while the difference disappears in the shade. The influence of colors on the absorbing powers of surfaces had already been made clear by the researches of Leslie and Melloni.
To form an estimate of the part which the conductibility proper of the different materials plays in these phenomena, M. Krieger inquired how much the loss of caloric was diminished when the cylinder was covered with double layers of the same cloths. The doubling of the satin, cotton cloth, and fine linen diminished the loss only by from three to six per cent, while doubling the envelopes of buckskin, flannel, and woolen cloth diminished it by ten, twenty, and even thirty per cent. It is clear from these experiments that the resistance offered by cloths to the passage of heat depends much less on the conductibility of the fibers that form their substance than on the thickness, the volume, and the texture of the tissues. This can also be shown in observing the cooling of a cylinder covered with wadding, which is forty per cent more rapid when the wadding is strongly compresesd. So a dressing-gown lined with wadding and a flannel waistcoat are warmer when we first put them on than after they have been worn for some time. The packing which the filaments undergo with use renders the cloth more permeable to heat. Although doubling the envelope has little influence when both layers are stretched tight over the cylinder, it is not the same when a slight space is left between them; then the cooling is retarded thirty or thirty-five per cent by the interposed stratum of air. Hence, we should expect in many cases to find a loose garment warmer than a tight one; and we know that close-fitting gloves or shoes afford but a poor protection against the cold. This reasoning, however, supposes that the protecting layer of air is motionless; but more frequently an ample and flowing garment favors the circulation of air, and therefore seems to us to be cooler, and is for that reason preferred in summer and in hot climates. We are now brought to the important fact that the most serious obstacle to the propagation of heat in any body is the discontinuity of its elements. This is because heat is a mode of motion, and every derangement of molecular continuity impedes the transmission of vibrations. This principle is more or less unwittingly put to profitable use in the manufacture of clothing. We obtain very warm clothes from light, loose, and porous tissues, having a capacity to retain in the spaces between their fibers a large volume of air. I said, retain; I might more properly have said, let pass; for the air which our clothes inclose is not motionless, but circulates and undergoes constant renewal in filtering through the envelopes which we mistakenly believe are intended to isolate us from the surrounding medium. It is, in fact, an essential condition of a good garment that it shall not interpose an obstacle to ventilation. The warmest clothes let the air pass more readily than those which are considered cool. Dr. Pettenkofer demonstrated this fact by measuring the volumes of air which under the same pressure and in the same time passed through a series of tubes stopped by pieces of different kinds of cloth. The numbers representing the volumes were for the different goods: flannel, 100; linen, 58; silk, 40; strong cloth, 58; buckskin, 51; glazed skin, 1. Flannel is, then, a hundred times more permeable to the air than a glazed glove, and we know at the same time that it is infinitely warmer. The volumes of air transmitted are but little changed by doubling the goods. Our clothes are thus continually aerated by an exchange, the activity of which depends on the external temperature, the degree to which the atmosphere is agitated, and the porosity of the tissues; the essential point is that the change shall be so slow that the nerves of touch shall not be affected by it. The warmest coat is one of fur, and its warmth lies not in the skin only, but chiefly in the hairs, although their mass is relatively insignificant, and is almost wholly due to the air interposed between them. Furs are warmer in proportion as the hairs are finer, because, doubtless, the air that circulates through them is more thoroughly warmed. There are formed around the bodies of furred animals superimposed strata of air, the temperature of which diminishes from the skin to the ends of the hairs; and in winter the animals seem cold to the touch, while the zone of exchanges retires toward the skin as the cold becomes more intense. The body of the animal is, then, cooled principally by convection and by the ventilation which incessantly removes the heated air. When the atmosphere is much agitated the cold penetrates more readily through the furs, and also through our overcoats, as all know who have been much exposed to cold winds. According to M. Krieger's experiments, the loss of heat through the skin is doubled when the fur is shaved off, and tripled when the skin is varnished. These facts bring us to the conclusion that the goods called impermeable are generally anti-hygienic, because they impede the aeration of the garments beneath them. They are good for protection against rain, but they excite perspiration and prevent its evaporation, and are very uncomfortable in pleasant weather.
Another very important property in cloths is their hygroscopicity; they condense moisture from the atmosphere and become impregnated with it the more speedily as the air is more nearly saturated with vapor, and consequently less capable of favoring evaporation. The condensation, which is equivalent to a kind of dew, is increased when the temperature is diminishing. According to M. Courier's researches, the water absorbed by a cloth may be divided into two parts: one part which is not perceptible to the touch and can not be pressed out—the hygrometric water proper; and the other part, that which fills the pores and can be wrung out, and which M. Coulier calls interposed water. According to his experiments, wool is more hygroscopic than hempen cloth, and linen than cotton. Dr. Pettenkofer compared the hygroscopic qualities of a piece of linen and a piece of flannel having equal surfaces and nearly equal weights. Having been previously dried at the boiling-point of water, the two pieces of goods were exposed together in places more or less moist, and the variations in weight they went through after several hours of exposure were measured. It was found that wool was nearly twice as hygroscopic as linen. Similar differences between different materials may be observed when they are wet by immersion. Linen gets wet much more speedily than wool, but the wool really absorbs the most water.
The quantity of water that cloths are capable of absorbing is evidently more considerable than is commonly supposed. A woolen coat weighing five or six kilogrammes may take up nearly a litre of water, and this will add a kilogramme to its weight. We see also that cloths absorb more moisture when the temperature is low than at ordinary summer heat. Wet garments conduct heat better than dry ones, and consequently give much less protection against chills; hence the danger of cold combined with dampness. But wool, although it is more hygroscopic than linen, protects better against the effects of humidity because of the slowness with which it absorbs and gives off water, and because of its indestructible porosity.
As we fill up the meshes and pores of a tissue, it becomes less permeable to air, and goods with close meshes, like linen, cottons, and silks, feel this effect much more quickly than woolen goods. As Dr. Pettenkofer remarks, the elasticity of the fibers counts for much in the persistence of porosity. The fibers of wool, even when moistened, lose but little of their elasticity and do not allow the pores to close, while the filaments of linen, cotton, and silk become quite soft under the influence of moisture, and do not resist the invasion of the water. For this reason damp wool cools us much less than damp linen. A linen or silken shirt is cooler than a woolen one, because it more completely sponges off the sweat and exposes it to evaporation. These facts illustrate clearly the capital influence which space between the fibers exercises on the physical properties of cloths. A cloth must evidently be considered as a tissue formed of textile matter and air. The properties of the fibers themselves can give us only the most incomplete ideas of the physical effects which their assemblage would bring about. The arrangement of the fibers and the manner in which they are prepared are most frequently the important points. There is reason to believe that by looking along this road, still so little explored, we shall reach results that will permit us to make a better use of some of the innumerable textile materials which Nature has put at the disposition of our industry.
Hygienists, in speaking of different cloths, are generally contented with classifying them vaguely in the order of conductibility, and of designating by that word the greater or less facility they offer to the passage of heat. It is agreed that conductibility decreases in the following order: linen or hemp cloth, cotton, silk, and wool. Cloths made of linen, hemp, and cotton, are considered the coolest. They are readily moistened and cool the skin by both conductibility and evaporation. Linen, whether made of hemp or flax, is, says M. Bouchardat, of all substances destined for clothing, the one that most favors the affections resulting from the impression of moisture on the skin. But with many persons the coolness and pleasant feeling of linen are very highly appreciated as advantages.
Cotton cloth lets less heat escape, absorbs and retains a part of the perspiration, and cools less rapidly by evaporation; its use is generally more advantageous than that of linen. An opinion or prejudice prevails widely that cotton is less healthful than linen, based on the fact that, being a less perfect conductor and rougher, cotton irritates the skin more than linen does. Examined with the microscope, the fibers of cotton appear angular and stiff, while those of linen are round and supple. Cotton is not agreeable in cutaneous affections, but wool, in such cases being more hairy and warmer, would be still more disagreeable. This is the only condition, says M. Bouchardat, in which any other substance than very fine, well-washed, and well-worn linen can be worse than it. Aside from this, cotton cloth has the advantage over linen of being warmer in winter, and, in summer, of not exposing the body to the dangers of too rapid cooling. It should be used in preference to linen by inhabitants of cold and moist countries. Wool is still more irritating than cotton, on account of the stiffness of the hairs with which it bristles; but the excitation it produces becomes a therapeutic means whenever the skin needs a stimulant. Unfortunately, its use next to the skin may become the source of the infirmities for the cure of which it is indicated when too effeminate an education has caused us to contract the habit of wearing it too early and without sufficient reason. It may cause a grievous predisposition to colds, rheumatisms, and neuralgias, while, the habit once acquired, it can not be given up without danger. But the use of wool is precious in some countries and under some conditions of life.
Professor Brocchi, a writer well known for his investigations in malaria, attributes the good health and vigor of the ancient Romans to their habit of wearing coarse woolen clothes; when they began to disuse them, and to wear lighter goods and silks, they became less vigorous and less able to resist the morbid influence of bad air. It was first at about the time the women began to dress in notably fine tissues that the insalubrity of the Roman air began to be complained of. Dr. Balestra, in his book on the "Hygiene of the City of Rome and of the Campagna," admits that there may be some little truth in these views, although the increasing unhealthfulness of the Roman climate was chiefly explainable by the abandonment of cultivation and the physical degeneracy of the people in consequence of the general change in the manner of living. At any rate, woolen clothing has a right to be considered an excellent prophylactic in countries infected with malaria. "In the English army and navy," says Dr. Balestra, "the soldiers of garrisons in unhealthy places are obliged constantly to wear wool next to the skin, and to cover themselves with sufficient clothing, for protection against paludine fevers, dysentery, cholera, and other diseases." According to Patissier, similar measures have been found efficacious to guard the health of workmen employed on dikes, canals, and ditches, in marshy lands; while, previous to the employment of these precautions, mortality from fevers was considerable among them.
The hygienic properties of wool are due, first, to a slight roughness of the surface that excites the functions of the skin; and, secondly, to its porosity, which, as we have already explained, moderates the expenditure of caloric and prevents a too sudden cooling of the body. Dr. Balestra believes that flannel contributes to the elimination from the body of the paludine miasms, which have been absorbed by the pores, and also to rid it of the deposits that cause rheumatic affections. The hypothesis is confirmed by the singular connection that seems to exist in miasmatic regions between rheumatic and intermittent fevers. Furthermore, woolen goods arrest in their down a portion of the germs borne in by the air which thus reaches the skin filtered and purified; Dr. Balestra has proved by direct experiments in marshy regions that thick and hairy woolen garments have the filtering power that is here attributed to them. It is hardly necessary to add that such clothing, to afford real protection, should be frequently washed. Cotton is next to wool in value, and is preferable to linen, because it gives a gentle excitation to the skin. Silk also has a warm feeling, and might be substituted for flannel in the winter; but it could hardly be worn next to the skin in summer, on account of the excessive heat it provokes. Dr. Balestra insists that it is best for inhabitants of unhealthy countries never to go out without being provided with a woolen cloak or blanket, to be used in case the weather should change. The ancient Romans wore ample over-garments over their tunics, and never put them away. It is no less important to be well covered during the night; and precautions of this kind should be recommended to all who live in a swampy country. We are sometimes astonished when we see the natives of particularly warm countries enveloped in woolen, as the Arab in his burnoose, or the Spanish peasant in his tobacco-colored cloak. Such materials protect both against the rays of the sun and against the coolness of the night, and are excellent regulators of heat. It is dangerously imprudent to travel in southern countries without provision of warm clothing.
The hat completes the dress, as the roof crowns the house. It preserves the head from insolation and cold, and protects it against accidents. Without going so far as M. Bouchardat, who says that the best head-dress is none, we content ourselves with remarking that the hat should be light and well aired. According to M. Troupeau's experiments, conical and rounded head-coverings are cooler than flat ones, and preferable in hot countries.
The bed is not only a piece of furniture indispensable to secure repose; it is also, in fact, a dress for the night. Like other articles of clothing, it should be both warm and permeable to the air. The heat which the body gives off to the mattress and the coverings is continually taken away by the air that traverses them. "Beds designed to regulate the flow of heat," says Dr. Pettenkofer, "are with us thicker than the garments which clothe us during the day, for two reasons: first, because, the circulation being less active during rest and sleep, less heat is evolved; and, secondly, because the ascending currents cool us more rapidly in the horizontal than in the vertical position, where they rise from the feet to the head, passing over the whole body." The heat of the bed thus favors the peripheric circulation and assists the internal organs that have to keep up the calorification. To do without a bed for several days in succession is a great privation, not only because it deprives the limbs of rest, but also because of the troubles to the economy it induces. A too hot and too soft bed is also objectionable because it keeps the body in a condition of moisture that enfeebles the muscular system and reduces all the functions. Feather-beds are generally more noxious than useful. They are warm on account of the air they hold; but an air-mattress should be as warm. Birds also clothe themselves in heat when they sleep, by ruffling up their feathers, putting themselves in the shape of a ball, and surrounding themselves with the thick strata of air included within the filaments of their plumage. We purpose in another article to consider the relations of our habitations to the atmosphere.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.
