<>o PRODUCTION Sect. XIV. -6. 



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

 a darkim 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 electric fluid, as it pafles 

 through them ; for I fuppofe thefe fenfations are occafioned by 

 flight electric (hocks produced in the following manner. By 

 the experiments publimed by Mr. Bennet, with his ingenious 

 doubler of electricity, which is the greateft difcovery made in 

 thatfcience fince the coated jar, and the eduction of lightning 

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

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

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

 ner above defcribed, 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 5 and is at the fame inftant 

 difcharged through the nerves of tafle or of fight, and gives the 

 fenfations, as above defcribed, of light or of faporofity ; and on- 

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

 ulus of the electric fluid in fuddenly paffing through them. 



VI. Of the Senfe of Heat. 



There are many experiments in chemical writers, that evince 

 the exiftence 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 combination of others. Thus 

 from the combinations of metals with acids, and from thofc 

 combinations of animal fluids, which are termed fecretions, this 

 fluid matter of heat is given out amongft the neighbouring bod- 

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

 is abforbed from the bodies, that furround them ; whilfl in its 

 facility in palling 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 

 flate, and to find its equilibrium. 



There is no circumftance 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 procefTcs, 

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

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

 circumftances that occafion them. And in this the percep- 

 tion 



