Description of a Properly of Caoutchouc. 41 



soon as it has been fully exerted. Perhaps it will be said, 

 that the preceding experiment is conducted in a negligent 

 manner^ that a person who wishes for accuracy will not 

 trust his own sense of feeling in inquiries of this description, 

 but will contrive to employ a thermometer in the business. 

 Should the objection be started, the answer lo it is obvious; 

 for the experiment in its present state demonstrates the 

 reality of a singular fact, by convincing that sens«, which 

 is the only direct judge in the case, that the temperature of 

 a piece of caoutchouc may be changed by compelling it to 

 change its dimensions. The use of a thermometer deter- 

 mines the relative magnitudes of these variations, by refer- 

 ring the question of temperature to the eye : experiments of 

 this sort arc therefore of a mathematical nature, and afiord a 

 kind of knowledge with which we have nothing to do at 

 present ; i'or we are not inquiring after proportions, but en- 

 deavouring to establish the certainty of a fact, which may 

 assist in discovering the reason of the uncommon elasticity 

 observable in caoutchouc. My essay or letter appears to be 

 running into a long digression ; the subject must therefore 

 be resumed, and it will not be improper to premise the fol- 

 lowing simple experiment, in the present state of the inquiry, 

 because it seems capable of affording no inconsiderable de- 

 gree of insight into the plan which nature pursues in pro- 

 ducing the phoenomenon in question. 



JExperiment II. 

 If one end of a slip of caoutchouc be fastened to a rod of 

 metal or wood, and a weight be fixed to the otlier extremity, 

 m order to keep it in a vertical position, the thong will be 

 found to become shorter with heat and longer with cold. 

 The processes of heating, cooling, and measuring bodies are 

 so well known, that I need not enter into the minuter parts 

 of the experiment; it will be proper however to add, that 

 an incre-ase of temperature diminishes the specific gravity of 

 the Indian rubber, and a loss of heat occasions a contrary 

 eflectinit, as I have proved cxpcrimentaliy. The know- 

 ledge of the latter fact leads me to conclude, apparently on 

 reasonable crovmd?, that the pores or interstices of caout- 

 chouc 



