294 A Description of a Property of 



its constituent particles move freely amongst 

 themselves : but where there is motion, there is 

 void space; consequently Caoutchouc abounds 

 with innumerable 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 di- 

 mensions of the pores in a piece of 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 to the 

 first experiment ; the pores of it are therefore 

 diminished ; and the effort, which it exerts at 

 the time, arises from the mutual attraction of 

 the Caoutchouc and Caloric ; which attraction 

 causes an endeavour to enlarge the interstices 

 of the former for the reception of the latter ; 

 hence it happens that the thong contracts longi- 

 tudinally, according to the second experiment, 

 and the redundant caloric is absorbed in the 

 course of this operation, which again reduces 

 the temperature. The preceding explanation 

 agrees very well with the phenomenon, as i-ir 

 is stated in the beginning of this letter ; and the 



