On the Cooling Effect produced on Solids by Tension. 381 



force is exerted so as to pull them separate from each other. 

 But the cooling effect which takes place shows that more force 

 disappears than simply what is exerted by the weight ; for the 

 cooling effect is caused by the disappearance of force in the shape 

 of heat from the body itself. The force exerted by the weight 

 disappears in performing work against the cohesion of the par- 

 ticles of the body stretched. But what becomes of the force in 

 the form of heat which disappears from the body at the same 

 time ? It must be consumed in performing work of some kind 

 or other. The force exerted by the weight cannot be the cause 

 of the cooling effect. The transference of force from the weight 

 to the body may be the cause of a heating effect — an increase of 

 force in the body; but this transference of force to the body 

 cannot be the cause of a decrease of force in the body. If a 

 decrease of force actually follows the application of tension, the 

 weight can only be the occasion, not the cause of the decrease. 



In what manner, then, does the stretching of the body by the 

 weight become the occasion of its losing force in the shape of 

 heat ? Or, in other words, what is the cause of the cooling effects 

 which result from tension ? The probable explanation of the 

 phenomenon seems to be this : if the molecules of a body are 

 held together by any force, of whatever nature it may be, which 

 prevents any further separation taking place, then the entire 

 heat applied to such a body will appear as temperature ; but if 

 this binding force becomes lessened so as to allow further ex- 

 pansion, then a portion of the heat applied will be lost in pro- 

 ducing expansion. All solids at any given temperature expand 

 until the expansive force of their heat exactly balances the cohe- 

 siveforce of their molecules, after which no further expansion at the 

 same temperature can possibly take place while the cohesive force 

 of the molecules remains unchanged. But if, by some means or 

 other, the cohesive force of the molecules become reduced, then 

 instantly the body will expand under the heat which it possesses, 

 and of course a portion of the heat will be consumed in expan- 

 sion, and a cooling effect will result. Now tension, although it 

 does not actually lessen the cohesive force of the molecules of the 

 stretched body, yet produces, by counteracting this force, the 

 same effect; for it allows the molecules an opportunity of per- 

 forming work of expansion, and a cooling effect is the conse- 

 quence. If the piston of a steam-engine, for example, be loaded 

 to such an extent that the steam is unable to move it, the steam 

 in the interior of the cylinder will not lose any of its heat ; but 

 if the piston be raised by some external force, the molecules of 

 the steam will assist this force, and consequently will suffer loss 

 of heat in proportion to the amount of work which they perforin. 

 The very same occurs when tension is applied to a solid. Pre- 



