n$ PRODUCTION OF IDEAS. SECT. XIV. 6. 



is attracted by the folations of fome of them, and 

 is detruded from the combination of others. Thus 

 from the combinations of metals with acids, and 

 from thofe combinations of animal fluids, which 

 are termed fecretions, this fluid matter of heat is 

 given out amongft the neighbouring bodies ; and 

 in the folutions of falts in water, or of water in 

 air, it is abforbed from the bodies, that furro\$nd 

 them ; whilft in its facility in paffing through me- 

 tallic bodies, and its difficulty in pervading refins 

 and glafs, it refembles the properties of the electric 

 aura ; and is like that excited by friction, and 

 feems like that to gravitate amongft other bodies 

 in its uncornbined (late, and to find its equili- 

 brium. 



There is no circum fiance of more confequence 

 in the animal economy than a due proportion of 

 this fluid of heat -, for the digeftion of our nutri- 

 ment in the ftomach and bowels, and the proper 

 qualities of all our fecreted fluids, . as they are pro- 

 duced or prepared partly by animal and partly by 

 chemical proceifes, depend much oh the quantity 

 of heat ; the excefs of which, or its deficiency, alike 

 gives us pain, and induces us to avoid the circum- 

 ftances that occafion them. And in this the per-r 

 ception of heat effentially differs from the percep* 

 tions of the f enfe of touch, as we receive pain from 

 too great preffure of folid bodies, but none from 

 the abfence of it. It is hence probable, that nature 

 has provided us with a fet of nerves for the percep* 

 tion of this fluid, which anatorniits have not yet 

 attended to. 



There may be fome difficulty in the proof of this 

 afiertion ; if we look at a hot fire, we experience 

 no pain of the optic nerve, though the heat along 

 with the light muft be concentrated upon it. Nor 

 does warm water or warm oil poured into the ear 

 give pain to the organ of hearing j and hence as 



thefe 



