Science of Dress/Chapter III
CHAPTER III.
HEAT.
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IN the last chapter I described the action of the lungs in purifying and supplying oxygen to the blood; but I could not do more than just touch upon the work which that oxygen does in the blood, in producing heat. Before I pass on to consider this, moreover, it will be well to say something about heat in general, for animal heat is subject to general laws which apply throughout nature, and which should be better understood by the majority of people than they are at present.
Our sensations respecting heat and cold are somewhat deceptive if their origin is not comprehended, and they have strengthened the popular belief that heat and cold are two material substances which enter into our bodies; but that this idea is fallacious will presently appear. The nerves of the skin, by which we know whether we feel hot or cold—whether heat is entering or leaving our bodies to a more than ordinary extent, do not enable us exactly to distinguish degrees of heat, as the eye distinguishes degrees of light, or the ear of sound.
Heat is a force which is found everywhere, and exists in every body, whether animate or inanimate. It may even be obtained from two pieces of ice by friction, which we know melts them. Cold is simply a relative term, which has no meaning in itself, but is used to signify low degrees of heat; thus, if we say, "Ice is very cold," we imply, "Ice has very little heat."
That every object must possess a certain degree of heat is evident if we regard heat as a mode of motion among particles, for everything in nature is composed of groups of particles. Tyndall, in his great work on "Heat as a Mode of Motion," explains this difficult question very clearly. "It seems possible," he says,1[1] "to account for all the phenomena of heat, if it be supposed that in solids the particles are in a constant state of vibratory motion, the particles of the hottest bodies moving with the greatest velocity, and through the greatest space. . . . Temperature may be conceived to depend upon the velocity of the vibrations; increase of capacity in the motion being performed in greater space." Thus friction produces increase of heat, by increasing the rapidity of the motion of particles, and every case of combustion "may be ascribed to the collision of atoms which have been urged together by their mutual attractions." The particles of all substances have a tendency to be set in motion by those of substances of a different kind, although "vibrations never take place between substances of the same nature." By this means motion is propagated among the particles of neighbouring bodies, by what is called diffusion of heat, until each body is of the same temperature, as may be shown by testing with the thermometer.[2] Thus, if we touch something, the temperature of which is higher than that of our bodies, an increased motion of our particles is set up, and we say it feels warm; whereas, if we touch something of a lower temperature, our own temperature is lowered by the contact, while that of the other body is raised. The vibration of the particles of bodies may be increased by percussion, friction, and combustion, or other chemical change.
Heat, like light, radiates from all bodies in straight lines, and the temperature of a body, on which radiant heat falls, is raised by transmitted motion, just as a string vibrates when a sound in unison with it is transmitted to it through the air. When heat passes by direct contact from particle to particle of a body, or from the particles of one body to another, it is said to be communicated by conduction, whereas, if a space intervenes it is said to pass by 'radiation. Thus the sun's heat is radiated on to the earth, the heat of a fire radiated to one sitting by it; but the heat of a poker held in the hand is conducted along the poker from the end held in the fire to that held in the hand.
Although all substances conduct heat, they do so with different rapidity; those that conduct it quickly are called conductors, while those through which its passage is slow are called non-conductors. For example, metals are good conductors; but wood, ivory, and animal substances, such as wool or hair, are bad conductors, or, as they are generally called, non-conductors. It is owing to this fact that substances, which are really of the same temperature, may appear to the touch to be of quite different degrees of warmth. A bit of wood, a piece of cloth, and a stone, lying side by side, will, according to the facts just mentioned, be of the same temperature, but the cloth will feel warmest, the stone coldest, because the cloth does not rapidly conduct away the heat of the hand, whereas the stone does.
Similarly air, although of the same temperature, as shown by the thermometer, will feel colder or warmer to us according to whether it is in motion or still; because when in motion it removes heat from our bodies quicker than when at rest, since more of it passes over our surface. Wind, as we know, is air in motion, and we feel colder on a windy than on a calm day, although the thermometer may register the same degree. Nevertheless, air is a bad conductor of heat: compare it, for instance, with water, which scalds at 150° Fahr.; whereas a man in a Turkish bath can without danger enter a room, the air of which is heated to 200°. Both air and liquids have heat imparted to them by a process called convection, in which heated particles, as they become lighter by expansion, rise, forming an upward current, while colder particles, being heavier, sink; but, becoming heated, rise again to give place to others. Thus a sort of circulation is kept up. Solid bodies communicate heat by conduction, because their particles, although vibrating, cannot change their material arrangement and position, the heat or vibrating motion simply passing from particle to particle, whereas the particles of liquids and gases are free to move, separate from each other, and, as Tyndall says, penetrate "in right lines through space."
I have said that substances tend to a uniform temperature with their surroundings, and that two bodies, such as a stone and a piece of cloth, will be found, other things equal, to be of the same temperature. When we come to speak of living things, however, whether vegetable or animal, although according to the laws of heat, there is the same tendency to uniformity, there is here a counteracting agency at work, and every living thing maintains a temperature peculiar to itself, and dependent on its own vital constitution, a temperature a rise only a few degrees above, or a fall only a few degrees below, which would cause its death.
During long-continued frost a thermometer in the centre of a tree trunk does not sink to freezing point. The temperature of the inside of a tree is said to be above that of the atmosphere if the latter be below 57° Fahr.; but if the temperature of the atmosphere rises above that point, that of the tree does not rise in the same proportion. Hunter found that when the bulb of a thermometer was introduced into the stomach of a carp, the mercury rose to 69°, although the temperature of the pond in which the fish swam was only 65½°. When the temperature of the air was 58°, that of a viper's stomach was found to be ten degrees higher; but when the viper was placed in a temperature of 108°, the heat of its stomach did not exceed 92½°. In what are called warm-blooded animals the difference between the temperature of the body and its surroundings is still more marked, the blood being sometimes nearly 100° above the temperature of the surrounding atmosphere. In the animal body, as long as there is life, the processes of nutrition, of which I have already spoken, are going on, and in the course of these processes heat is constantly being evolved by chemical changes going on in the blood, which is the nourishing fluid. The oxygen gas breathed into our lungs is absorbed into the blood, it there meets with various compounds of carbon derived from the food, and uniting chemically with these gives off heat and produces the carbonic acid which we breathe out, as previously described. Rather less carbonic acid is given off from the lungs than the oxygen absorbed, and it is supposed that the remainder of the oxygen enters into combination with hydrogen in the blood, producing some of the water which is given off from the body, and also with compounds of hydrogen, nitrogen, and carbon, thus further increasing the animal heat, for heat is evolved whenever chemical combination takes place. Oxygen is carried in the arterial blood all over the body, and wherever there are capillaries, there the oxygen combines with the carbon, and heat is evolved, as elsewhere—in a grate, for instance. But it must be remembered that something is necessary to set up the combination. In the grate, we apply it in the form of a lighted match, and in the same way a certain degree of temperature is required in the body. The combination of oxygen with carbon is going on all over the body wherever the two meet—not only in the lungs, as some have been led to suppose, because it is from the lungs that the chief amount of carbonic acid, the result of their union, is given off.
The origin of animal heat has been for centuries a debated question, and until of late no thoroughly satisfactory results have been obtained; but the theory here explained rests upon facts gained by numerous scientific experiments, and upon inductions therefrom. There is therefore every reason to believe that it is the true one, although animal heat may be in part caused by the friction of the blood in its rapid passage through the arteries and veins, as well as by the chemical action. The theory is borne out by the fact that when the circulation is slow, as in old people and during sleep, the temperature is lowered; whereas in fevers, where the temperature is high, the pulse is always found beating rapidly, and the breathing is quickened.
We know that a fire will not burn unless the air can get to it—that is to say, unless oxygen can come to combine with the carbon of the coal; it is the same in the animal body: the carbon which is derived from the food eaten is, as it were, the fuel which sets our machinery in motion; but that it is not wholly expended in work has been proved by experiment and observation. The muscles convert chemical force into mechanical work, but to quote the words of Dr. Mayer, "The maximum mechanical effect produced by a working mammal hardly amounts to one-fifth of the force derivable from the total quantity of carbon consumed; the remaining four-fifths are devoted to the generation of heat." When a muscle contracts, heat is developed in it. This has been found so even in the muscles of dead frogs. And in the case of persons who die of that terrible disease tetanus, in which a general contraction of the muscles takes place, and death is caused either by starvation through lockjaw, or suffocation, owing to the prolonged contraction of the muscles employed in breathing, the temperature of the muscles is sometimes found to be nearly 11° Fahr. above the normal. The arterial blood charged with oxygen when passing through an un-contracted muscle is changed into venous blood, which then retains about 7½ per cent, of oxygen; but if the muscle is contracted, the arterial blood is almost wholly deprived of its oxygen, the quantity remaining in some cases only amounting to 1 3/10 per cent. When the muscles are in activity, therefore, there is increased combustion, more fuel is burnt up. As a consequence of this an increased amount of carbonic acid is expired from the lungs, and during great exertion the quantity of this gas which is expired may be even five times as much as that breathed out during repose. A further consequence of the increased combustion, moreover, is that an increase of food is required, and we all know how hungry vigorous exercise makes us.
It is noticeable, too, that the colder the temperature of the air is, the quicker combustion goes on in the body to keep up its heat; breathing is then performed more rapidly; more oxygen, too, is contained in cold than in warm air, and more carbon is required to combine with it in the blood. Thus a larger quantity of food is rendered necessary. In the far North the natives consume an amount of food, and especially of fatty carbonaceous foods, which seems fabulous to those who do not understand why it is required. It is said that an Esquimaux boy can drink two quarts of train oil and eat twelve pounds of tallow candles in a day; or whale blubber corresponding to that amount.
The foregoing satisfactorily explains the reason why exercise of muscle or brain makes us hungry, for brain work uses up the blood to give nourishment, and evolves heat just as muscular work does, and it also shows why we feel that we want more food, and especially more carbonaceous—more fatty food in cold than in hot weather. Whether a body is animate or inanimate, if it has a temperature above that of its surroundings, as has been shown, it gives away more heat to them than it can receive from them, and it thus continually grows cooler in proportion to the difference between the two temperatures and to the degree of its exposure. As the temperature of the animal body is, except under very rare conditions, always a great deal higher than that of the surrounding atmosphere, animals are constantly losing heat, and it becomes a matter of vital importance that some non-conducting substance shall be interposed between the warm blood and the cold air.
All warm-blooded animals are endowed by nature with some protection against the constant loss of heat. Thus seals, whales, porpoises, and other warm-blooded animals living much in water are protected by a layer of fat through which water cannot pass, and which resists the passage of heat. Land animals also have this protective fat, though in a less degree. Moreover, the skin, being to a certain extent non-conducting, partially prevents excessive loss of heat, and in this duty it is supplemented in the lower animals by feathers or fur, and in man by clothes. Animal substances, such as hair, fur, wool, and feathers, are non-conductors of heat, and the colder the habitat of the creature the thicker the feathers or fur which cover it.
Look, for example, at the thick fur of an Arctic bear, or the feathers of the grebe, which are closely laid over one another to the height of about an inch and a half above the skin. These are nature's protection to the creatures on which they grow, and they save those creatures from being frozen to death, keeping their blood very far above the temperature of the surrounding atmosphere, which may be below freezing-point, while the interior of their bodies is about 99° to 104° Fahr. Thus human travellers in Arctic regions case themselves in fur, as, without such a non-conductor to retain it, the heat of their bodies would be conducted away into the freezing air quicker than it could be replaced, and the coldness of death would inevitably follow.
Probably every one is aware that clothes are worn to keep us warm, but not all seem to know that they do not communicate warmth to us, but effect their purpose simply by preventing the excessive loss of that heat which is manufactured in our own bodies. Hence, we are warm in proportion as our clothes are non-conductors of heat. As an instance of ignorance on this point, I may quote from an article called " Practical Hints on Children's Clothing," published in The Queen shortly after the publication in that paper of my own views on the subject. I should not have referred to this article, which is for the most part merely a restatement of those popular ideas to which I take exception, had not "A Lover of Babies" made the following remark. She says, "It must not be forgotten that wool is a non-conductor of heat, so that if you wrap a child up in ever so much flannel and do not first ascertain that its body and legs are warm, it remains cold all day."
Now, the temperature of a baby's body is nearly 100° Fahr., while that of the room in which the baby spends most of its time should not exceed 65° Fahr., and the little one is often in much lower temperatures than that; but my critic apparently believes that warmth gets into the body from the outside, ignoring the above fact, and missing the whole object of my contention, that children should be clad in woollen to prevent the loss of their natural heat. In point of fact, a shivering baby put into thick woollen clothes would very soon become warm and comfortable, simply because it would thus be allowed to retain the heat it was making for itself. Again, "A Lover of Babies" speaks of fur jackets as "producing undue heat," and thus only restates the popular fallacy.
Heat has been made the subject of so many thousand books, lectures, and articles by scientific men, that it is really time every one should know that it is the body which makes animal heat, not the clothes. We may find a homely but convincing proof of this fact any cold night when we get into bed. We are at first chilled by the contact with the sheets, but the chill wears off, and, when we rise, the housemaid finds the bed quite warm. It has become so from the heat of our bodies, which, leaving it, was kept from passing into the outer air by the non-conduction of the blankets, and of the feathers or hair with which the bed or mattress was stuffed.
Since clothes are only a supplement to the skin, being to man what wool, fur, and feathers are to other creatures, the most natural substances for human wear are the spoils of those other creatures. From the earliest times man has robbed the beasts of their skins in order to increase the protective power of his own less efficient heat-retaining cuticle. The book of Genesis gives great importance to this ancient custom, and speaks of the Deity as Himself making garments out of skins for Adam and Eve to wear. Thus, in Semitic belief; as, in point of fact, the study of savage life teaches us, the first clothes were skins, and through all the thousands of years which have elapsed since primitive man first clothed himself in finery borrowed from his slaughtered prey to the present day, fur has been a favourite garb of shivering mortality.
The invention of weaving, however, the origin of which dates so far back that it is shrouded in a mist of ages which history has not lifted, brought woollen materials and those made of vegetable fibre into rivalry with the more easily obtained fur. This was a great advance. To produce woven garments no animals had to be slaughtered, while woollen made serviceable ordinary wear, being a good non-conductor, more adaptable than skins, and capable of being washed. Clothes made of vegetable fibres were found pleasant in hot weather, as, being good conductors, they allowed the heat of the body to pass away from it, thus giving a sensation of coolness.
Besides their adorning functions, clothes have two purposes—to keep the body warm and to cover it. Primitive men and women in cold weather wore their skin garments; but in warm weather, discarding these, they probably considered themselves sufficiently attired in a little paint and a few feathers. Later on in civilization, however, it came to be considered decorous to keep the body always covered. In cold weather this was a pleasure, and furs and woollens were worn with satisfaction; but in sultry heat these became a burden, and then it was that garments woven out of vegetable fibre were welcomed as a relief, for vegetable fibres are good conductors of heat, so that garments woven out of them feel cool to the body—at any rate, when first put on.
The savage was doubtless more comfortable in his summer undress, but the next best thing to it was a dress of linen or cotton, which only slightly hinders the loss of heat from the body that takes place when it comes in contact with air cooler than itself.
It is evident from the foregoing that animal substances, especially woollen, form the best and most natural clothing, except in hot weather, and about this subject I shall have more to say hereafter.