H 



and Pyroelectrio Properties of Matter. 19 



20. The general thermodynamic principles expressed above in 

 the equations (6), (8), (12), and (13) enable us to determine 

 the relations between the evolution of heat or cold by strains 

 of any kind effected on an elastic solid, the variation of its 

 elastic forces with temperature, and the differences and varia- 

 tions of its specific heats. Thus (6) gives at once, when the 



development of w expressed by (18) is used, and for l-j—) &c. 



are substituted P &c, which are infinitely nearly equal to them, 

 the following expression for the heat absorbed by an infinitely 

 small straining, namely from (# , y w z w f , %, Q to (x, y, z, 



* f—dP; w -dQ, Y . ~dR r . 



-S(^)-S^>-f «-»}• • < 22 > 



21. We conclude that cold is produced whenever a solid is 

 strained by opposing, and heat when it is strained by yielding 

 to, any elastic force of its own, the strength of which would 

 diminish if the temperature were raised — but that, on the con- 

 trary, heat is produced when a solid is strained against, and 

 cold when it is strained by yielding to, any elastic force of its 

 own, the strength of which would increase if the temperature 

 were raised. When the stress is a pressure, uniform in all di- 

 rections, fluids may be included in the statement. Thus we 

 may conclude as certain: — 



(1) That a cubical compression of any elastic fluid or solid 

 in an ordinary condition would cause an evolution of heat ; but 

 that, on the contrary, a cubical compression would produce 

 cold in any substance, solid or fluid, in such an abnormal state 

 that it would contract if heated, while kept under constant 

 pressure. 



(2) That if a wire already twisted be suddenly twisted 

 further, always, however, within its limits of elasticity, cold 

 will be produced ; and that if it be allowed suddenly to untwist, 

 heat will be evolved from itself (besides heat generated exter- 

 nally by any work allowed to be wasted, which it does in un- 

 twisting). For I suppose it is certain that the torsive rigidity 

 of every wire is diminished by an elevation of temperature. 



(3) That a spiral spring suddenly drawn out will become 

 lower in temperature, and will rise in temperature when sud- 

 denly allowed to draw in. [This result has since been experi- 

 mentally verified by Joule (" Thermodynamic Properties of 

 Solids," Trans. Roy. Soc. 1858), and the amount of the effect 



C2 



