It has been proved with tolerable certainty that gases, such as carbonic acid and oxygen, are capable of penetrating and permeating the skin in small quantities, but it is extremely doubtful if water is ever absorbed through the skin. It has been attempted to settle the question by weighing the body before and after a prolonged immersion in the water, but such experiments are so beset with fallacies that they are almost worthless. The fact that shipwrecked sailors are in the habit of successfully lessening their thirst by immersion of the body in water, or by wetting their clothes, is well known, but this effect may be due to the arrest of the cutaneous evaporation, or by an effect upon the nerves. At all events it seems safest, in the present state of our knowledge, to assume that water is not absorbed through the skin; or if it be, that it is absorbed in such extremely small quantities that the effect of baths can in no sense be due to the absorption of the water in which the body is immersed. As to the absorption of the various salts contained in sea-water or mineral waters, there is no evidence whatever that these are ever absorbed even in the most minute quantities. If the salt dissolved in sea-water were absorbed through the skin, it is tolerably certain that sea-bathing, far from being the luxury which it is, would be regarded as a highly dangerous and most unpleasant practice. Baths of all kinds serve, or may be made to serve, as vehicles for temperature, and by their aid we are enabled to surround the body with a temperature which is different to its own. Before we can fully understand the effect of hot and cold baths on the economy, it is necessary to enter into some discussion of the nature and source of the natural heat, of the body. The natural heat of the human body is between 98° and 99° of Fahrenheit’s scale; and this temperature, roughly speaking, is uniformly maintained by the healthy body under all the varying circumstances to which it may be subjected. In the arctic regions, and in the tropics the temperature of the body rests at 98·6°; or, if variations occur, they are so slight in amount as to be hardly noticeable. In a cold atmosphere, therefore, the body has the power of maintaining its heat; and in a warm atmosphere it is equally able to maintain its coolness. This is a remarkable fact, and is due to the power possessed by the human body of adjusting the production and loss of heat. Heat is produced in the body by the combustion of food and tissues, exactly as heat is produced in a fireplace by the combustion of coal. The amount and rapidity of this combustion necessarily varies with the amount and nature of the food consumed and the activity of exercise and other vital processes. The most active tissue in the body is the blood; through its agency most of the combustion processes are carried on, and by its rapid circulation to all parts of the body the most distant points of the human frame are kept at the same