and Pyroelectric Properties of Matter. 2 1 



few pounds weight on a common india-rubber band, and taking 

 a red-hot coal, in a pair of tongs, or a red-hot poker, and 

 moving it up and down close to the band. The way in which 

 the weight rises when the red-hot body is near, and falls when 

 it is removed, is quite startling. Joule experimented on the 

 amount of shrinking per degree of elevation of temperature, 

 with different weights hung on a band of vulcanized india- 

 rubber, and found that they closely agreed with the amounts 

 calculated by my theory from the heating effects of pull, and 

 cooling effects of ceasing to pull, which he had observed in 

 the same piece of india-rubber. Joule's experiments leave 

 the statements of the following paragraph (5) true for common 

 india-rubber at 5° C, but reverse it for common india-rubber 

 at higher temperatures and for vulcanized india-rubber — that 

 is to say, leave it applicable to these substances with " pull " 

 substituted for "push" throughout.] 



(5) We may conclude as highly probable, that pushing a 

 column of india-rubber together longitudinally while leaving it 

 free at its sides will cause the evolution of heat, when the force 

 by which its ends are pushed together falls short of a certain 

 limit : but that, on the contrary, if this force exceeds a certain 

 limit, cold will be produced by suddenly increasing the force a 

 very little, so as to contract the column further. For I suppose 

 it is certain that a column of india-rubber with no weight, or 

 only a small weight on its top, will expand longitudinally when 

 its temperature is raised ; but it appears to me highly probable 

 that if the weight on the top of the column exceed a certain 

 limit, the diminished rigidity of the column will allow it to de- 

 scend when the temperature is raised. [This second change 

 we now know to be contrary to the true state of the case ; for 

 we have seen that the rigidity of india-rubber is augmented by 

 elevation of temperature.] 



22. The specific heat of an elastic solid homogeneously strained 

 under given pressures or tensions will be obtained by finding 

 the differential coefficients of oc, y, z, £, 77, t, with reference to 

 t, so as to make P, Q, R, S, T, U each remain constant or vary 

 in a given manner — that is to say, by finding the coefficients 

 of expansion in various dimensions for the body with an infi- 

 nitely small change of its temperature, and using these in (3) 

 above. 



23. The elastic properties of such a crystal as is frequently 

 found in natural specimens of garnet — a regular rhombic do- 

 decahedron — must, if they correspond to the crystalline form, 

 be symmetrical with reference to six axes in the substance 

 perpendicular to the six pairs of opposite faces of the dodeca- 

 hedron, or to the six edges of a regular tetrahedron related to 



