The most important object of dress from a hygienic standpoint is to regu late the amount of heat that is given off from the skin. Clothing limits the amount of heat lost in cool surroundings, and allows the ready passage of the heat which is constantly formed in the body. But the human body not only gives up heat to the surrounding air, but water, in the form of invisible vapour, as well. The amount of aqueous vapour given off varies with the temperature, the muscular activity, and the amount of humidity of the atmosphere. H large quantities of water are given off, it appears, not in the form of vapour, but in drops, as perspiration. The clothing should not offer any resistance to this giving off of water ; and for this reason it should be porous, so as to allow the passage of air. Undoubtedly great differences are to be poted in the heat-retaining properties of the various fabrics.
Experience teaches that a woollen shawl retains the body heat better than a cotton or linen one. The property of being able to retain the body heat is not due to the substance of the fabric, but is rather a property of its mode of weaving. A right proportion between the amount of substance and the size of the mesh determines the quality with reference to the retaining of heat and moisture. A second factor is found in the thickness of the fabric. In keeping with their lesser air content, the smoothly woven fabrics retain the heat the least ; loosely woven materials more, and flannel fabrics the best. There is a very large amount of air space (about of the whole) in flannel fabrics.
Although it follows that the heat-retaining quality of the various fabrics is determined especially by the nature of the weave rather than of the sub stance, there are certain qualities Nrhich do not render them equally suitable to be manufactured into textures. Wool can be woven into the loosest textures, whereas vegetable fibres are less well adapted for this purpose. If the conditions of looseness and of thickness are of especial importance when it is a question of protection from loss of heat in cold surroundings, the porosity of the fabric is of particular moment in hot temperatures. Prob ably everyone has experienced the oppressing sensation of being overheated during the hot summer months \Nile n the air is very moist. How does this sensation arise ? The body rids itself of its own heat by conduction, by radiation, and by the evaporation of water. The loss of heat by the evapora tion of perspiration is especially important. Restriction of this function leads to an overheating of the body ; sometimes even to an accumulation of heat with dangerous consequences. See HEAT-STROKE. The evapora tion of water from the skin is retarded especially when, in addition to the customary outer garments, closely woven fabrics such as smooth linen or cotton textures are worn as undergarments. As already mentioned, these contain a comparatively small amount of air ; hence, they do not retain heat. This of itself would be an advantage during the hot weather. But this advantage is more than outweighed by their inferior porosity, since such textures offer considerable resistance to the evaporation of water from the skin. This causes a ready formation of perspiration, which soon clogs up the few air-pores of the garment and completely hinders further evaporation.
But this is not all. The linen or cotton texture, fully saturated with the water of perspiration, no longer contains any air ; hence, it does not retain heat at all. This condition does not cause any inconvenience so long as the individual remains in a heated atmosphere, but he will feel the annoy ance thereof the moment he passes into a cooler place. Experience has
taught the necessity for being cautious when cooling off after strong perspira tion. Anyone dressed in garments of loose texture will in such an instance run much less risk of catching a cold than a person who wears clothes of smooth texture. The latter, rendered airless by the accumulation of sweat, act in an extremely cooling and refrigerating manner. The fabric of loose texture may also absorb sweat, but this soon evaporates through the pores of the garment, and it is only in the rarest cases that all the pores become filled with water. Sufficient air will almost invariably be retained to pro tect the skin from too sudden loss of heat, and, therefore, from excessive • exposure.
The following general principles regarding a rational mode of dressing can be deduced from the above statements. The essential point is the correct selection of undergarments. These should be made of a loosely woven fabric, which should be selected heavier in winter, lighter in summer. Smooth, closely woven linen and cotton fabrics are not suitable ; the starched bosom shirts used by men are especiallt out of place, as the starching renders them absolutely unporous. It is not sufficient, however, to secure loose, porous underclothing, as the favour able effects provided by it can be counterbalanced by closely woven, unpor ous outer garments. The smooth, thin. woollen fabrics which are preferably worn as outer garments in summer arc harmless in this respect, as they are still sufficiently porous ; the same applies to the more loosely constructed fabrics. But the material used for linings is generally quite unfavour able for ventilation, and its removal or displacement by other, more porous material would be desirable. With changeable temperatures. a sensibly dressed person regulates his requirement of heat and cold by a correspond ing change in his outer garments, wearing the same underclothing. With regard to undergarments the question remains to be decided whether loosely woven woollen, cotton, or linen fabrics should be preferred. As is well known, everyone of these materials has had its advocate in its time. Jager extolled woollen garments as the only correct ones ; Lahmann defended cotton ; and Pastor Kneipp favoured linen fabrics. The discussion already outlined shows that a so-called " normal dress," suitable for all conditions. and based upon a certain elementary substance. does not exist. The only rational dress is one which is adapted to the changing external and bodily conditions. However, it cannot be denied that wool possesses certain advantages over the other elementary substances. The fine little fibres of wool act as supporting hairs, creating a kind of insulating layer of air between the skin and the undergarment. Some persons, however, cannot wear wool next to the body because their skin is very sensitive. Wool absorbs water only incompletely, and therefore remains *drier ; it absorbs the secretions of the skin well, and it is lighter than garments made of vegetable fibres. Other mystic qualities, such as are ascribed to it by Jager, wool does not possess. On the other hand, these superiorities of woollen garments are counterbalanced by certain disadvantages. 'Woollen fabrics are soon worn out, and they soon become dirty. To combine the favourable properties of animal and vegetable fibres, mixed fabrics are now manufactured which are well adapted for porous underclothing.