20 SirW. Thomson on the Therrfioelastic, Thermomagnetic, 



found to agree with that calculated, according to the prece- 

 ding thermodynamic theory, from the amount of the weaken- 

 ing of the spring which he found by experiment.] 



(4) That a bar or rod or wire of any substance with or with- 

 out a weight hung on it, or experiencing any degree of end 

 thrust, to begin with, becomes cooled if suddenly elongated by 

 end pull or by diminution of end thrust, and warmed if sud- 

 denly shortened by end thrust or by diminution of end pull: 

 except abnormal cases, in which, with constant end pull or end 

 thrust, elevation of temperature produces shortening ; in every 

 such case pull or diminished thrust produces elevation of tem- 

 perature, thrust or diminished pull lowering of temperature. 



(4/) That an india-rubber band suddenly drawn out (within 

 its limits of elasticity) produces cold, and that, on the con- 

 trary, when allowed to contract, heat will be evolved from it. 

 For it is certain that an india-rubber band with a weight 

 suspended by it will expand in length if the temperature be 

 raised. [Alas for overconfident assertion ! This is not true — 

 at all events not true in general for either natural or vulcanized 

 india-rubber, but only true for india-rubber in somewhat ex- 

 ceptional circumstances. It was founded on the supposition that 

 india-rubber becomes less rigid when raised in temperature, 

 which, besides, seeming to be expectable for solids generally, 

 seemed to be experimentally proved for india-rubber by the 

 familiar stiffness of common india-rubber in very cold weather. 

 My original supposition is in fact correct for india-rubber 

 which has become rigid by being kept at rest at a low tempe- 

 rature for some time. In this condition india-rubber was 

 found by Joule to be cooled when suddenly stretched, and 

 heated when the stretching weight was removed ; and therefore, 

 when in this condition, it is certain, from the thermodynamic 

 principle, that a band of the substance bearing a weight will 

 expand in length if the temperature is raised, and shrink when 

 the temperature is lowered. But the very piece of india- 

 rubber in which Joule found a cooling effect by pull when its 

 temperature was 5° C, gave him a heating effect by pull, and 

 a cooling effect on withdrawal of pull, when the temperature 

 was 15° C. Joule experimented also on vulcanized india- 

 rubber, and with it always found a heating effect when the 

 substance was pulled out, and a cooling effect when it was 

 allowed to shrink back. I pointed out to him that therefore, 

 by thermodynamic theory, a vulcanized india-rubber band, 

 when stretched by a constant weight of sufficient amount 

 hung on it, must, when heated, pull up the weight, and when 

 cooled, allow the weight to descend. This is an experiment 

 which any one can make with the greatest ease by hanging a 



