The invisible and insensible perspiration can become visible and sensible when either its evaporation into the air is checked, as in the case of tight-fitting clothing, such as a rubber overcoat, or when the temperature of the medium around the body is raised. Both conditions can be fulfilled by fur clothing, for which reason its use as wearing apparel is to be deprecated. When in a state of perspiration we should take special care that the passage of the perspiratory products into the air shall not be impeded by any means. Fur garments are not so unsuitable for old people in whom the perspiratory functions of the skin are much diminished. We have, in this connection, observed an old lady of 76 sitting at the dinner table, close to an overheated fireplace, and wearing a large fur collarette, in which she felt quite comfortable. As frequently mentioned in this book, in old age there is a degeneration of the thyroid gland to a greater or lesser degree, and consequently a lowering of the temperature of the body and a diminution of the functions of the skin, which is dry and rarely perspires. When the temperature of the medium which surrounds the body is raised to a high degree, the capillaries of the skin are widened and more blood circulates through them; thus also more blood is conveyed to the sudorific glands, and naturally more water is then excreted by them, and at the same time also more of the solid and harmful products. According to Camerer, the sweat contains in each 100 grammes 50.6 g. ashes, 10 g. fat, and 10 g. nitrogenous matters, of which 30 per cent. is urea and 75 per cent. is ammonia. Uric acid was also found in the sweat of healthy people, but Magnus-Levy found it missing in gouty patients. By frequent sweating procedures the hygienic conditions of the skin can be promoted, and especially so in individuals whose skin is generally dry and inactive. In those who perspire much too freely, however, after several procedures of this kind the overactivity of the sudorific glands can be diminished. After very free perspiration the condition of the skin will be improved, and then we will note the disappearance of certain skin diseases, such as psoriasis in hot summer weather.