SICT. XIV. 6. PRODUCTION OF IDEAS 127 



if their oppdfite edges are then brought into con- 

 tact before the point of the tongue, a tafte is per- 

 ceived at the moment of their coming into con- 

 tact ; fecondly, if one of the above plates be put 

 between the upper iip and the gum of the fore- 

 teeth, and the other be placed under the tongue, 

 and their exterior edges be then brought into con- 

 tact in a darkifh room, a flafli of light is perceived 

 in the eyes. 



Thefe effects I imagine only mew 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 mocks produced in the following manner. 

 By the experiments publifhed by Mr. Bennet, with 

 his ingenious doubler of electricity, which is the 

 greateft difcovery made in that fcience fince the 

 coated jar, and the eduction of lightning from thd 

 fkies, it appears that zinc was always found minus, 

 and filver was always found plus, when both of 

 them were in their feparate ftate. Hence, when 

 they are placed in the manner above defcribed, as 

 foon as their exterior edges come nearly into con- 

 tact, fo near as to have an extremely thin plate of 

 air between them, that plate of air becomes charg- 

 ed in the fame manner as a plate of coated glafs $ 

 and is at the fame inltant difcharged through the 

 nerves of tafte or of fight, and gives the fenfations, 

 as above defcribed, of light or of faporofity ; and 

 only mews the great fenfibility of thefe organs of 

 fenfe to the itimulus of the electric fluid in fudden- 

 ly pafling through them., 



VI. Of the Senfe of Heat. 



THERE are many experiments in chemical writ- 

 ers, that evince the exiftence of heat as a fluid 

 element, which covers and pervades all bodies, and 



is 



