870. The chief excretion performed by the skin, in the human body, is commonly known under the name of perspiration. The perspiration is either sensible or insensible. Sensible perspiration is the liquid commonly called the sweat. Insensible perspiration consists of a vapour which, under the ordinary circumstances in which the body is placed, is invisible. The invisible vapour is constantly exhaling; the visible liquid is only occasionally formed. The quantity of matter carried out of the system under the form of invisible vapour is much greater than that lost by the visible liquid. 871. That a quantity of matter is incessantly passing off from the surface of the skin, under the form of an invisible vapour, is proved by the following facts:— 1. If the hand and arm are enclosed in a glass jar, the inner surface of the glass soon becomes covered with moisture. 2. If the tip of the finger be held at about the twelfth of an inch from a mirror, or any other highly polished surface, the surface rapidly becomes dimmed by the vapour which condenses upon it in small drops, and which disappear on the removal of the finger. 3. If the body be weighed at different periods, an accurate account being taken of the ingesta and the egesta, it is found to undergo a loss of weight sensibly greater than can be attributed to any of the visible discharges: this loss must be owing to the transmission of a quantity of matter out of the body, under the form of invisible vapour. 872. The matters excreted under the form of perspiration are separated from the blood by a true and proper secretion, like the other secretions of the body. The process by which this is effected is called transudation. The matter of transudation deposited on the surface of the skin by a vital function is removed from the body by evaporation, a physical process which consists of the conversion of a liquid into a vapour by the addition of heat. Consequently the process of perspiration is a cooling process, and it is chiefly by the increase of the perspiration that the body is enabled to bear the intense degrees of heat which it has been shown (491, _et seq._) to be capable of sustaining. Sitting one day in repose in the shade during the intense heat of an American summer’s day, the skin freely perspiring at every pore, Dr. Franklin happened to examine the temperature of his body with a thermometer. He found that the temperature of his body was several degrees lower than that of the surrounding air. The physiologists who exposed themselves in heated chambers, for the sake of ascertaining the greatest degree of heat which the human body is capable of enduring, perspired profusely during the experiment (495). The artisans who carry on their daily occupations in elevated temperatures perspire most profusely (884, _et seq._). Under such circumstances, caloric is communicated to the human body just as freely as to inorganic matter