Manchester Memoirs, Vol. Iv. (191 1), No. \%. 5 



It is apparent that if, on the extension of rubber, first 

 one effect and then the other is dominant, the resulting 

 thermal effects will, at some point, undergo a change in 

 sign, and for a certain extension there will be a neutral 

 point or point of inversion where there will be neither 

 heating nor cooling. At this point the heat energy 

 engendered by the motion of the molecular groups 

 relatively to one another, is equal to the potential energy 

 gained on account of the enlargement of the inter- 

 molecular spaces. This point of neutrality, Chauveau 

 states, exists both during extension and retraction, but 

 not at the same tensions. 



It is important to note that the two effects cannot be 

 detected at the ordinary temperatures of the atmosphere, 

 but are only found at low temperatures. Perhaps this is 

 due to the fact that, at the higher temperatures, the weak 

 type of molecular group breaks up at the first application 

 of stress, or, it is possibly unstable altogether at the 

 ordinary temperatures of the atmosphere. Under these 

 conditions the extensibility of the rubber is much greater 

 than at low temperatures, and it would seem that the 

 positive work is in excess of the negative work done for 

 all loads. As the temperature is lowered, a point is 

 reached at which the two kinds of work done are equal in 

 amount at the commencement of the application of 

 load, and below this point the negative work would 

 predominate initially. 



The classic experiments of Joule on this subject were 

 conducted at 6"C. and 7'8°C. in the case of unvulcanised 

 and vulcanised rubber respectively, but it does not appear 

 that he took any great care to keep his temperature 

 constant, and it is quite possible that even a small 

 variation in temperature would affect the result, since 

 Joule says, "At temperatui'es a few degrees higher, the 



