"W E say of one piece of goods that it is warm, of another that it is cool. What do we mean by that? Have fur and woolen cloth a heat of their own that they communicate to us? Do we demand of wool, down, or cotton some supplementary heat furnished by their own substance?
"Not at all; for none of these materials, be it the silkiest down or the softest fur, has any heat of its own, and they cannot supply it to us. Their office is limited to preventing the loss of such heat as is within us, the natural heat that our bodies generate simply by living. Garments and wraps are poor conductors interposed between the human body, warmed by the vital heat, and exterior objects which, colder than the body, would otherwise lower its temperature. They are to us what a shovelful of ashes is to the firebrands on the hearth. They do not warm us, but they preserve our natural heat; they do not add anything to us, but they prevent loss."
"Then a garment that we call warm," said Marie, "is not really any warmer than another, but only keeps the heat of the body in better!"
"That is it, exactly. You see, apart from all idea of finery and pleasing adornment, the value of a garment from the sole point of view of real usefulness in retaining heat depends above all on its poor conducting power. The worse conductor it is, the better the garment will fill its office. In this respect wool heads the list of materials used in our textile goods; offering the most resistance to the radiation of the body's heat, it is the most serviceable in protecting us from cold. Silk, cotton, hemp, and flax are far inferior to it. And that is as it should be. Wool is the sheep's clothing; therefore its attributes are assigned to it by Providence for protecting from cold the chilly animal that wears it. Silk, cotton, hemp, and flax have in the order of nature other uses: silk insures the safety of the caterpillar and its chrysalis in the cell of a firmly woven cocoon; the feathery down of the cotton-plant causes the seeds to float in the air and enables them to travel long distances and germinate in far-off places; the mission of hemp and flax is to strengthen long and fragile stems with their fibers. What was not clothing by nature became so by man's art, but it never acquires to the same degree the valuable qualities that belong to wool, the natural clothing of the animal it covers.
"Cotton in its turn conducts heat less readily than hemp or flax; hence cotton goods are preferable to linen for underwear that comes next to the skin and by its direct contact with us is so effective in maintaining our temperature. With cotton, sudden chills and arrested perspiration, with all their dangerous consequences, are less to be feared than with linen.
"Best of all substances air is the poorest heat-conductor. Therefore we envelop ourselves, as it were, with air. Our textiles of wool, cotton, etc., are in a way only a sort of network for retaining the air in their innumerable meshes. This layer of air maintained all round our body protects us from cold the more effectively the thicker it is and the less liable to renewal. Hence it is not the heaviest and closest-woven fabric that is the warmest, but supple, soft stuff that receives an abundance of air throughout and holds it captive in its meshes, as do cotton-wool and down.
"Furthermore, between the body and its clothing, and retained by the latter, is found an envelop of air which we must take into consideration since it constitutes a natural lining that nothing could replace. To play its part well this lining of air must be of a certain thickness, which is assured by our wearing clothing that is loose enough but not too loose, because in the latter case the air, being renewed too easily, would be constantly replaced by cold air and, reversing its function, would cause a loss of heat instead of a gain."
"I must confess, Uncle Paul," Marie interrupted again, "I had no idea air played any such part in our clothing. Who would have imagined that we keep ourselves warm by holding a little stale air around us! Doubtless it is the same with our bed-covering!"
"Precisely the same. Our bed-covers and hair mattresses are only barriers preventing the loss of natural heat. The light feathers, wool, cotton, hair composing them retain the air in abundance in their fluffy mass and thus form a non-conducting envelop which the heat of the body cannot pass through.
"Mattresses are even made of air alone without any fibrous material whatever. A substantial casing of canvas fastened in a frame is kept in shape by means of springs arranged inside. This pouch is filled with a layer of air, which is both soft and yielding, so as to insure comfort, and very effective in retaining heat. It is far preferable to heavy straw mattresses, being cleaner, less cumbersome, and easier to handle."