344 HAND-BOOK OF PHYSIOLOGY. 



the skin which passes off by evaporation; the sweat includes that which 

 may be collected only in drops of fluid on the surface of the skin. The 

 two terms are, however, most often used synonymously; and for distinc- 

 tion, the former is called insensible perspiration; the latter sensible per- 

 spiration. The fluids are the same, except that the sweat is commonly 

 mingled with various substances lying on the surface of the skin. The 

 contents of the sweat are, in part, matters capable of assuming the form 

 of vapor, such as carbonic acid and water, and in part, other matters 

 which are deposited on the skin, and mixed with the sebaceous secretion. 



Table of the Chemical Composition of Sweat. 



Water 995 



Solids: 



Organic Acids (formic, acetic, butyric, pro- ) .^ 



pionic, caproic, caprylic) j 



Salts, chiefly sodium chloride . . . 1 *8 



Neutral fats and cholesterin . . . ? 



Extractives (including urea), with epithelium 1-6 5 



1000 



Of these several substances, however, only the carbonic acid and water 

 need particular consideration. 



Watery Vapor. The quantity of watery vapor excreted from the 

 skin is on an average between 1^- and 2 Ib. daily. This subject has been 

 estimated very carefully by Lavoisier and Sequin. The latter chemist 

 enclosed his body in an air-tight bag, with a mouth-piece. The bag 

 being closed by a strong band above, and the mouth-piece adjusted and 

 gummed to the skin around the mouth, he was weighed, and then re- 

 mained quiet for several hours, after which time he was again weighed. 

 The difference in the two weights indicated the amount of loss by pul- 

 monary exhalation. Having taken off the air-tight dress, he was imme- 

 diately weighed again, and a fourth" time after a certain interval. The 

 difference between the Wo weights last ascertained gave the amount of 

 the cutaneous and pulmonary exhalation together; by subtracting from 

 this the loss by pulmonary exhalation alone, while he was in the air-tight 

 dress, he ascertained the amount of cutaneous transpiration. During a 

 state of rest, the average ftss by cutaneous and pulmonary exhalation in 

 a minute, is eighteen grains, the minimum eleven grains, the maximum 

 thirty-two grains; and of the eighteen grains, eleven pass off by the skin, 

 and seven by the lungs. 



The quantity of watery vapor lost by transpiration is of course influ- 

 enced by all external circumstances which affect the exhalation from 

 other evaporating surfaces, such as the temperature, the hygrometric 



