40 Description of a Property of Caoutchouc. 



The property I am about to describe depends on the tem- 

 perature of the caoutchouc which is used in the experi- 

 ment; for heat increases the pHancy of the substance, and 

 cold, on the contrary, renders it more rigid ; so that when 

 a shp of this resin has been sufliciently warmed, it inay be 

 extended to more than twice its natural length, by a moderate 

 force applied to its extremities; after which it will recover its 

 ori'jinal dimensions in a moment, provided one of the ends of 

 it be let go as soon as it h.i? been stretched. This disposition 

 of the substance may be produced by a degree of temperature 

 less than the heat of the blood ; it is therefore necessary to 

 prepare a slip of it, by steeping it for a few minutes in warm 

 water, or by holding it somewhat longer in the fist : either of 

 these precautions makes the resin pliant, and lits it for the 

 experiment, which is perfoimed in the followine; manner: 



I made a pince of caoutchouc a little heavier than an equal 

 bulk of water, the temperature of which was 43 degrees : 

 the vessel containing the resin and water was then placed on 

 the fire; and when the contents of it were heated to 130 

 degrees, the caoutchouc floated on the surface. 



Experiment I. 

 Hold o-nc end of the slip, thus prepared, between the 

 thumb and fore-finger of each hand, bring the middle of the 

 piece into slight contact with the edges of the lips, taking 

 care to keep it straight at the time, but not to stretch it 

 much beyond its natural length : after taking these prepara- 

 tory steps, extend the slip suddenly, and you will immedi- 

 ately perceive a sensation of warmth in that part of the 

 mouth which touches it, arising from an augmentation of 

 temperature in the caoutchouc ; for this resin evidently 

 grows warmer the fiu-ther it is extended, and the edges of 

 the lips possess a high degree of sensibility, which 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 ins-tant, by permitting the slip to 

 contract again ; which it will do quickly by virtue of its 

 own spring, as oft as the stretching force ceases to act as 



soor; 



