SEC. 4. THE NATURE AND AMOUNT OF PERSPIRATION. 



§ 350. The quantity of matter which leaves the human body- 

 by way of the skin is very considerable. Thus it has been esti- 

 mated that while -5 gram passes away through the lungs per 

 minute, as much as *8 gram passes through the skin. The 

 amount, however, varies extremely; it has been calculated, 

 from data gained by enclosing the arm in a caoutchouc bag, 

 that the total amount of perspiration from the whole body in 

 24 hours might range from 2 to 20 kilos; but such a mode of 

 calculation is obviously open to many sources of error. 



Of the whole amount thus discharged, part passes away at 

 once as watery vapour mixed with volatile matters, while part 

 may remain for a time as a fluid on the skin; the former is fre- 

 quently spoken of as insensible, the latter as sensible perspiration 

 or sweat. The proportion of the insensible to the sensible per- 

 spiration will depend on the rapidity of the secretion in refer- 

 ence to the dryness, temperature and amount of movement of 

 the surrounding atmosphere. Thus, supposing the rate of 

 secretion to remain constant, the drier and hotter the air, and 

 the more rapidly the strata of air in contact with the body are 

 renewed, the greater is the amount of sensible perspiration which 

 is by evaporation converted into the insensible condition ; and 

 conversely when the air is cool, moist, and stagnant, a large 

 amount of the total perspiration may remain on the skin as sen- 

 sible sweat. Since, as the name implies, we are ourselves aware 

 of the sensible perspiration only, it may and frequently does 

 happen that we seem to ourselves to be perspiring largely, when 

 in reality it is not so much the total perspiration which is being 

 increased as the relative proportion of the sensible perspiration. 

 The rate of secretion may, however, be so much increased, that 

 no amount of dryness, or heat, or movement of the atmosphere, 

 is sufficient to carry out the necessary evaporation, and thus the 

 sensible perspiration may become abundant in a hot, dry air. 

 And practically this is the usual occurrence, since certainly a 

 high temperature conduces, as we shall point out presently, to 

 an increase of the secretion, and it is possible that mere dryness 

 of the air has a similar effect. 



550 



