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LXI. On the Cause of the Cooling Effect produced on Solids by 

 Tension. By Mr. James Croll*. 



FROM a series of experiments made by Dr. Joule with his 

 usual accuracy, he found that when bodies are subjected 

 to tension, a cooling effect takes place. " The quantity of cold," 

 he says, "produced by the application of tension was sensibly 

 equal to the heat evolved by its removal; and further, that the 

 thermal effects were proportional to the weight employed" - !** He 

 found that when a weight was applied to compress a body, a 

 certain amount of heat was evolved ; but the same weight, if ap- 

 plied to stretch the body, produced a corresponding amount of 

 cold. 



This, although it does not appear to have been remarked, is a 

 most singular result. If we employ a force to compress a body, 

 and then ask what has become of the force applied, it is quite a 

 satisfactory answer to be told that the force is converted into 

 heat, and reappears in the molecules of the body as such • but if 

 the same force be employed to stretch the body, it will be no 

 answer to be told that the force is converted into cold. Cold 

 cannot be the force under another form, for cold is a privation of 

 force. If a body, for example, is compressed by a weight, the 

 vis viva of the descending weight is transmitted to the molecules 

 of the body and reappears under that form of force called heat ; 

 but if the same weight is applied so as to stretch or expand the 

 body, not only does the force of the weight disappear without 

 producing heat, but the molecules which receive the force lose 

 part of that which they already possessed. Not only does the 

 force of the weight disappear, but along with it a portion of the 

 force previously existing in the molecules under the form of heat. 

 We have therefore to inquire, not merely into what becomes of the 

 force imparted by the weight, but also what becomes of the force 

 in the form of heat which disappears from the molecules of the 

 body itself. That the vis viva of the descending weight should 

 disappear without increasing the heat of the molecules is not so 

 surprising, because it may be transformed into some other form 

 of force different from that of heat. For it is by no means evi- 

 dent a priori that heat should be the only form under which it 

 may exist. But it is somewhat strange that it should cause the 

 force previously existing in the molecules in the form of heat 

 also to change into some other form. 



When a weight, for example, is employed to stretch a solid 

 body, it is evident that the force exerted by the weight is con- 

 sumed in work against the cohesion of the particles, for the entire 



* Communicated by the Author, 

 t Phil. Trans, for 1859, p. 91. 



