Many people have already shortened their existence by catching colds. Pneumonia, pleurisy, nephritis, and many other dangerous ailments have arisen from such a cause and led to premature death. A good hygiene of the skin not only serves to eliminate toxic products from the body and keep our kidneys in good order, but also prevents the tendency to catching cold, which is so often the cause of an early death. We must, therefore, do our best by a careful rational attention to the hygiene of the skin, and for this purpose we will deal further with the subject in two chapters on the hygiene of the skin by bathing and by rational clothing. ------------------------------------------------------------------------ CHAPTER XXV. ON RATIONAL CLOTHING. THE chief object of rational clothing is to obtain porosity. All underwear as well as outer coverings must be made out of some porous substance which will in no way check the perspiration from the body or the evaporation of such perspiration, and which will at the same time allow of a free passage of air to all parts of the body. First, we will discuss the question as to the nature of the underwear which is most appropriate for keeping ourselves in a healthy condition. Wool is the most suitable material for warmth for underclothing, as it is a bad conductor of heat and can therefore best retain the natural heat of the body. It follows, therefore, that this is the best for aged persons to use. In old age, especially when very advanced, less natural heat is produced in the body. On account of the degenerative changes of the small capillaries the surface of the body is not so well provided with the heating element, which is the blood. At the same time, on account of the loss in elasticity of the musculature, the skin of an aged person contracts less readily from cold than does the skin of younger people; thus, while on the one hand less warmth is produced, on the other hand more warmth may be given off, a due consideration of which leads to the logical conclusion that all aged persons had best wear woolen underclothing. Woolen underwear, though the best to retain the warmth of the body, has, on the other hand, very serious drawbacks. Wool possesses the great advantage of easily absorbing the perspiration of the body; it can, in fact, absorb about 40 per cent. of the moisture, by weight, without becoming so saturated as to be noticeable; but at the same time it gives off this moisture again very slowly, necessitating the changing at once of such damp covering. The greatest drawback to wool, however, is that it soon loses its porosity after it has been washed, for then it shrinks and its meshes contract, and in consequence the material is no longer porous; while in this condition it does not freely give off the moisture, but retains it, the aëration of the skin is seriously hindered, and the products of perspiration are not gotten rid of.