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CHAP. II.] continually removing matters from the blood. For instance, if a man's body, or even only one limb, is carefully enclosed in a gutta-percha bag full of air, it is found that changes take place in the air similar to those which happen in respired air. The air loses oxygen and gains carbonic acid, and it also receives a quantity of watery vapour, which condenses on the side of the bag, and may be drawn off if desired. Ordinarily such moisture does not appear on the surface of the skin, but it is given off nevertheless, and is then called insensible perspiration; but when the external temperature is very high, or under certain circumstances of mental emotion, or violent bodily activity, it appears in the form of isolated drops irregularly distributed on the skin, and thus becomes sensible perspiration. The air in hot climates is far from being wholly saturated with vapour of water, and in temperate climates it ceases to be so directly it touches the skin, the temperature of which is generally twenty or thirty degrees above its own.

The quantity of sweat, or perspiration, whether sensible or insensible, varies immensely, according to conditions of the external air, and to states of the blood and nervous system. It is calculated that as a general rule the skin gives off about two pints in twenty-four hours, nearly double as much water as the lungs do in the same time, but not more than one-thirtieth or fortieth part as much carbonic acid as they do, while it gives off about one and a half per cent, of solid saline matter, lactic acid, urea, and other waste products which would act as