2 Schwartz & Kemp, Physical Properties of Rubber. 



edges of the lips and then suddenly extending it he 

 experienced a sensation of warmth arising from an 

 augmentation of the temperature of the rubber, and then 

 by allowing the strip to contract again he found that this 

 increase of temperature could be destroyed in an instant." 



In his next experiment he found that " If one end of 

 a slip of Caoutchouc be fastened to a rod of metal or 

 wood, and a weight be fixed to the other extremity, in 

 order to keep it in a vertical position ; the thong will 

 be found to become shorter with heat and longer with 

 cold." 



In his third experiment he says, " If a thong of 

 Caoutchouc be stretched, in water warmer than itself, it 

 retains its elasticity unimpaired ; on the contrary, if the 

 experiment be made in water colder than itself, it loses 

 part of its retractile power, being unable to recover its 

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

 while it remains extended for want of spring, and the heat 

 will immediately make it contract briskly." 



Joule^ also observed the curious fact that a piece of 

 indiarubber, softened by warmth, may be exposed to the 

 zero of Fahrenheit for an hour or more without losing its 

 pliability, but that a few days' rest at a temperature con- 

 siderably above the freezing point will cause it to become 

 rigid. 



The remarkable series of experiments carried out by 

 Joule on the thermo-dynamic properties of rubber will 

 be referred to in detail in connection with the authors' 

 experiments on the same subject. 



Thermo-Dynamic Properties. 



When a solid body is subjected to a tensile force, 

 certain molecular changes take place, the mechanism of 



■^ " Some Thermo-dynamic Properties of Solids."— /%«/. Trans. ^ 1859. 



