J)escriptionof a Properly of Caautchouc. 43 



to the first experiment: the pores of it are therefore aimi- 

 nished ; and the effort which it exerts at the time arises from 

 the mutual attraction of the caoutchouc and caloric, which 

 attraction caus-s an endeavour to enlarge the interstices of 

 the former for the reception of the latter : hence it happens 

 that the thong contracts longitudinally, according to the 

 second experiment, and the redundant caloric is absorbed in 

 the course of this operation, which again reduces the tem- 

 perature. The preceding explanation agrees very well with 

 the phcenomenon, as it is stated in the beginning of this 

 letter ; and the theory receives additional confirmation from 

 the following facts. 



Experiment III. 

 If a thong of caoutchouc be stretched, in water warmer 

 than itself, 'w. retains its elasticity unimpaired ; on the con- 

 trary, if the experiment be made in water colder than itself, 

 it loses part of its retractile power, being unable to recover 

 its former figure ; hut let the thong be placed in hot water, 

 while it remains extended for Vv'ant of spring, and the heat 

 will immediately make it contract briskly. The foregoing 

 circumstances may be considered as proving, that the elasti- 

 city of caoutchouc is not a constitutional quality of the 

 substance, but a contingent effect, arising from the loss of 

 equilibrium between the portion of caloric, which the resin 

 happens to contain at any moment, and its capacity to re- 

 ceive that fluid at the same instant. The object of the pre- 

 sent letter is to demonstrate, that the faculty of this body- 

 to absorb the calorific principle may be lessened, by forcibly 

 diminishing the magnitudes of its pores, and this essential 

 point of the theory may be confirmed by experiment; for 

 the specific gravity of a slip of caoutchouc is increased, by 

 Keeping it extended while it is weighed in water. 



John Gditgh. 



VIII. Ucmar'ks 



