Ramie is a material made from a tropical plant, the _Bæhmeria Nivea_, and has lately come much into use. It readily absorbs the moisture from the skin and does not retain it as long as does wool, but rapidly gives it off again. It also does not abstract warmth so much as linen, and next to wool is the warmest material for underwear. The drawbacks to it, however, are that it is too heavy and does not last long. Silk underwear also retains the warmth, and also very easily absorbs moisture; but it is too expensive for ordinary use, and can readily be spoilt by indifferent washing, unless the most expensive quality is used. Of all the various materials for use as underwear possibly cotton has the greatest average advantages. It gives off the moisture it has absorbed from the skin, although not quite so efficaciously as does linen. According to James Paton it absorbs moisture equally well as linen; but Pettenkoffer is of a different opinion, as in his view cotton does not absorb the moisture from the skin as readily as does linen. The greatest advantage, however, of cotton over all other kinds of material (except perhaps ramie) for underwear is its porosity. It is about the most porous material there is, especially if in the course of manufacture the maker takes particular care in selecting the very best cotton; it does not shrink like wool, and therefore does not lose its porosity in the same way when washed. When cotton is so manufactured as to present the greatest possible porosity, it has also the great advantage of being almost as warm as wool, which is due to the fact that air can enter freely everywhere and remain in the interstices of the material. It has already been mentioned that air is a bad conductor of heat, so that cotton underwear of good quality retains the warmth of the body and at the same time allows a free exit for the exhalations of the skin. If, therefore, cotton be prepared from the best possible material, and manufactured in such a manner as to obtain the greatest amount of porosity, i.e., when it is loosely woven, it can compete successfully with wool as material for warm underwear; it is, therefore, quite an erroneous view to hold, as so many do, that cotton is very different in this respect from wool; everything depends upon the _quality_ and the manner in which it is manufactured. According to the researches of Sir William Thompson[235] there is practically no difference between wool, cotton, and linen in regard to their capacity as conductors of heat. Nevertheless we ourselves still believe that in winter weather, or when there are cold northern winds, wool is best, especially for the aged. This same holds good also for those younger persons who catch cold easily. Footnote 235: Sir William Thompson, in “Heat,” Encyclopædia Britannica.