42 Description of a Property of Caoutchouc. 



chouc are enlarged by heat and diminished by cold ; conse- 

 quently when a. slip of this substance which remains extended 

 by a weight, or the application of force, happens to contract 

 from an accession of temperature, the capacity of its pores, 

 taken separately or collectively, is augmented by the change 

 that takes place in the figure of the thong. Now if the ex- 

 istence of caloric be admitted, it will follow from the pre- 

 ceding arguments, that thephaenomenon under consideration 

 is occasioned bv the alternate absorption and emission of the 

 calorific fluid, in the same manner that ropes, the blades of 

 fuel, as well as many more bodies, are obliged to contract 

 and extend themselves, by the alternate absorption and 

 emission of water. You will perceive by the tenour of the 

 foregoing observations, that my theory of this case of elas- 

 ticity is perfectly mechanical ; in fact, the explanation of it 

 depends upon the mutual attraction of caloric and caout- 

 chouc ; the former of which penetrates the latter, and 

 pervades every part of it with the greatest ease and expedi- 

 tion, by which the resin is compelled to accommodate its 

 pores to that portion of the calorific fluid which is due to its 

 ^A-hole mass, at any particular degree of temperature. In 

 order to apply the last remark to the ph^enomenon under 

 ■consideration, I may observe, that if a force be exerted on a 

 piece of caoutchouc to alter the dimensions of its pores, the 

 mutual attraction mentioned above will resist the effort. But 

 the ease with which this substance may be made to change 

 its figure, and the retractile power which it possesses on 

 these occasions, show that its constituent particles move 

 freely amongst themselves : but where there is motion there 

 is void space ; consequently caoutchouc abounds with innu- 

 merable pores 'or interstices, the magnitudes of which are 

 variable, because the specific gravity of the resin becomes 

 less with heat and greater with cold. Now if the dimensions 

 of the pores in a piece uf caoutchouc can be lessened, 

 without taking away part of the matter of heat which it 

 contains at the time, this new arrangement in the internal 

 structure of the slip will lessen its capacity for the matter of 

 heat, and consequently augment its temperature. But the 

 warmth of such a slip is increased by stretching it, according 

 8 tp 



