CLASS!, i. 2. 14. OF IRRIGATION. 25 



manner as the fecretion of tears is defigned to preferve the cor- 

 nea of the eye moift, and in confequence transparent ; yet has 

 this cutaneous mucus been believed by many to be an excre- 

 ment ; and I know not how many fanciful theories have been 

 built on its fuppofed obftruction. Such as the origin of catarrhs, 

 soughs, inflammations, eryfipelas, and herpes* 



To all thefe it may be fufficient to anfwer, that the ancient 

 Grecians oiled themfelves all over ; that fome nations have 

 painted themfelves all over, as the Pi6ts of this ifland ; that the 

 Hottentots fmear themfelves all over with greafe. And laftly, 

 that many of our own heads at this day are covered with the 

 flour of wheat and the fat of hogs, according to the tyranny of a 

 filthy and wafteful fafhion, and all this without inconvenience. 

 To this muft be added the ftricl: analogy between the ufe of the 

 perfpirable matter and the mucous fluids, which are poured for 

 fimilar purpofes upon all the internal membranes of the body ; 

 and befides its being in its natural ftate inodorous ; which is 

 not fo with the other excretions of feces, Or of urine. 



The quantity of perfpirable matter being greater than that of 

 the excrementitious matters voided by ftool and urine, has been 

 ufed as an argument in favour of its being an excrement ; the force 

 of which I de not fee : but can readily underftand, that there 

 muft of neceffity be a great exhalation of a fluid which is difFu- 

 fed over the whole external furface of the warm fkirt, and per- 

 haps warmer lungs, for the purpofe of keeping them moift and 

 pliant, and which is perpetually renewed as it evaporates ; but, 

 if it be conceived to be an excrement, there feems to have been 

 no neceffity for its quantity being fo great. 



The evaporation of this great quantity of fluid, fecreted on the 

 farface of the fkin and lungs, muft carry off much heat from 

 the body ; and as both this fecretion and confequent evaporation 

 will be in proportion to the aclivity of the cutaneous veflels, and 

 the heat occafioned by their increafed fecretion, it would feem, 

 that this evaporation of perfpirable matter is the caufe which 

 preferves the animal body at the uniform degree of heat of 98 ; 

 in the fame manner as the evaporation of boiling water preferves 

 it at 2 1 2 degrees of Farenheit's fcale. 



The peculiar ufe of the perfpirable matter in preferving the 

 membranes moift, which line the air-pipes of the lungs, appears 

 from the curious difcovery of Dr. Prieftley, that the oxygen of 

 the atmofphere will pafs through moift animal membranes, but 

 not through dry ones, fo that if the membranes of the trachea 

 were to become dry, the animal muft as immediately perifh as- 

 if he was to breath azotic gas alone. See Sect. XXVITI. 2. cf 

 the preceding volume. 



VOL. II. E la 



