290 A Deseripiion of a Proptriij, of 



EXPERLMENT l. 



Hold one end of the slip, thus prepared, be- 

 tween the thum*b and fore-finger of each hand ; 

 bring, the middle of the piece into sliight contact 

 with the edges of the. iips ; taking care to 

 keep it straight at the time, but not to stretch 

 it much beyond its natural length : after taking 

 these preparatory steps, extend the slip sud- 

 denly ; and you will immediately perceive a 

 sensation of warmth in that part of the mouth 

 which touches it, arising from an augmenta- 

 tion of temperature in the Caoutchouc : for 

 this resin evidently grows warmer the further 

 it is extended ; and the edges of the lips possess 

 a high degree of sensibility, whic;;V} enables 

 them to discover these changes with greater 

 facility than other parts of the body. The 

 increase of temperature, which is perceived 

 upon extending a piece of Caoutchouc, may 

 be destroyed in an instant, by permitting the 

 slip to contract again 3 w^hich it will do quickly 

 by virtue, of its own spring, as oft as the 

 streching force ceases to act as sooti as it has 

 been fully exerted. Perhaps it \\\\\ be said, 

 that the preceding experiment is conducted in 

 a negligent manner; that a person, who wishes 



