308 



that Its capacity 

 may be mecha- 

 nically altered 

 and riie caloiic 

 extruded} &c. 



Overftretched 

 Caoutchouc 

 does not com- 

 pletely recover 

 itfelf in the 

 cold ; but heat 

 reftores its elaf- 

 ticity : 



whence the 

 nature of its 

 ' eUiticity is de- 

 duced, ££C. 



i»R r E R TY V i> fv -i 5f D J A N . R U: B B E R ^ ' 



af any particular degree of temperafure. In order to apply 

 the laft remark to the phenomenon under confideration I may 

 obferve, that if a force be exerted on a piece of Caoutchoue 

 to alter the dimcnfions of its pores, *ihe mutual attradion 

 mentioned above will refjll the effort. But the eafe with 

 which this fubllance may be m^ide to change its figure, and 

 the retratlile power which it poflTefles on thefe occafions, 

 fliew that its conftituent particles move freely amongft them- 

 ielves : but where there is motion, there is void fpace ; con- 

 fequently Caoutchouc abounds with innumerable pores or in- 

 terftices, the magnitudes of which are variable, becaufe the 

 fpecific gravity of the relin becomes lefs with heat, and greater 

 with cold. Now if the dimenfions of the pores in a piece 

 of Gaoutchouc can be lefiened, without taking away part of 

 the matter of heat, which it contains at the time; this new 

 arrangement in the internal ftru6ture of the flip will lefTen 

 its ca[)acity for the matter of heat, and confequently augment 

 its temperature. But the warmth of fuch a flip is increafed 

 by ftretciiing it, according to the firft experiment; the pores 

 of it are therefore diminiflied : and the effort, which it exerts 

 at the time, arifes from the mutual attraciion of the Caout- 

 chouc and Caloric ; which attradlion caules an endeavour to 

 enlarge the inlerfticcs of the former for the reception of the 

 latter ; hence it happens that the thong conlrafls longitu- 

 dinally, according to the fecond experiment, and the re- 

 dundant caloric is abforbed in the courfe of this operation, 

 which again reduces the temperature. The preceding ex- 

 planation agrees very well with the phenomenon, as it is fiated 

 in the beginning of this letter ; and the theory receives ad- 

 ditional confirmation from the following fa6ls. 



Exp. 3. If a thong of Caoutchouc be ftretched in water 

 warmer than itfelf, it retains its elaflicity unimpaired ; on the 

 contrary, if the experiment be made in water colder than it- 

 felf, it lofes part of its retradlile power, being unable to 

 . recover its former ligure ; but let the thong be placed in hot 

 water, while it remains extended for want of fpring, and 

 the heat will immediately make it contrail briftLly. The 

 foregoing circumftances maybe confidered as proving, that 

 the elafticity of Caoutchouc is not a conftilulional quality of 

 the lubflance, but a contingent effecl, arifing from the lofs 

 of equilibrium between the portion of caloric, which the 



re(iu 



