38 INSENSIBLE PERSPIRATION. 



constantly giving out a large quantity of waste ma- 

 terials by what is called insensible perspiration, a 

 process which is of great importance to the preser- 

 vation of health, and which is called insensible, 

 because the exhalation, being in the form of vapour, 

 and carried off by the surrounding air, is invisible to 

 the eye ; but its presence may often be made mani- 

 fest even to sight by the ner approach of a dry cool 

 mirror, on the surface of which it will soon be con~ 

 densed so as to become visible. 



Many attempts have been made to estimate accu- 

 rately the amount of exhalation carried off through 

 the skin ; but so many difficulties stand in the way 

 of obtaining precise results, and the difference in 

 different constitutions and even in the same person 

 at different times is so great, that we must be satis- 

 fied with an approximation to the truth. Sanctorius, 

 who carefully weighed himself, his food, and his ex- 

 cretions, in a balance, every day for thirty years, 

 came to the conclusion that Jive out of every eight 

 pounds of substance taken into the system passed out 

 of it again by the skin, leaving only three to pass off by 

 the bowels, the lungs, and the kidneys. The cele- 

 brated Lavoisier and M. Seguin afterward entered 

 on the same field of inquiry, and with greater suc- 

 cess, as they were the first to distinguish between 

 the cutaneous and pulmonary exhalations. M. Se- 

 guin shut himself up in a bag of glazed taffetas, which 

 was tied over his head and provided with a hole, the 

 edges of which were glued to his lips with a mixture 

 of turpentine and pitch, so that the pulmonary ex- 

 halation might be thrown outwards, and the cutane- 

 ous alone be retained in the bag. He first weighed 

 himself and the bag in a very nice balance, at the 

 beginning of the experiment ; then at the end of it, 

 when he had become lighter in proportion to- the 

 quantity of exhalation thrown out by the breathing ; 

 and, lastly, he weighed himself out of the bag, to 

 ascertain how much weight he had lost in all ; aad 



