9* PRODUCTION SECT. XIV. 6. 



tongue, and their exterior edges be then brought into contact in 

 a darkifh room, a flam of light is perceived in the eyes. 



Thefe effects I imagine only (hew the fenfibility of our nerves 

 of fenfe to very fmall quantities of the eleftric fluid, as it pafles 

 through them, for I fuppofc thefe fenfarions are occafioned by 

 flight electric (hocks produced in the following manner. By 

 the experiments publiuhed by Mr. Bennet, with his ingenious 

 doubler of eletricity, which is the greateft difcovery made in 

 that fcience fmce the coated jar, and the eduction of lightning 

 from the ikies, it appears that zinc was always found minus, 

 and (ilver was always found plus, when both of them were in 

 their feparate (late. Hence, when they are placed in the man- 

 ner above dcfcribed, as foon as their exterior edges come near- 

 ly into contact, fo near as to have an extremely thin plate of air 

 between them, that plate of air becomes charged in the fame 

 manner as a plate of coated glafs ; and is at the fame inftant 

 discharged through the nerves of tafte or of fight, and gives the 

 feniations, as above defcribed, of light or of faporofify , and on- 

 ly (hews the great fenfibility of thefe organs of fenfe to the ftim- 

 ulus of the electric fluid in fuddenly palling through them. 



VI. Of the Senfe of HeaU 



THERE are many experiments in chemical writers, that evince 

 the exigence of heat as a fluid element, which covers and per- 

 vades all bodies, and is attracted by the folutions of fome of 

 them, and is detruded from the combiation 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 neighboring bod- 

 ies \ and in the folutions of falts in water, or of water in air, it 

 is abforbed from the bodies, that furround them 5 whilft in its 

 facility in pafling through metallic 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 uncombined 

 ftate, and to find its equilibrium. 



There is no circumitance of more confequence in the animal 

 economy, than a due proportion of this fluid of heat \ for the 

 digeftion of our nutriment in the ftomach and bowels, and the 

 proper qualities of all our fecreted fluids, as they are produced 

 or prepared partly by animal and partly by chemical procefles, 

 depend much on the quantity of heat ; the excefs of which, or 

 its deficiency, alike gives us pain, and induces us to avoid the 

 circumft-ances that occafion them. And in this the percep- 

 tion 



