Pyro-electricity and Piezo-electricity of Crystals. 453 



geneous mass must have presented themselves in a very 

 striking manner at higher temperatures and under stronger 

 pressures, and at a certain stage resulted in the production of 

 the above optically homogeneous appearance. 



It is therefore very likely, as Jamin remarked, that we 

 have under those circumstances of temperature and pressure 

 nothing that is different in kind from what we see under 

 ordinary circumstances. 



Dewar's experiments, especially those on liquefaction of 

 carbonic acid in presence of other liquids, are interesting 

 from our point of view, and we cannot but think of some 

 such process as interdiffusion being in action through all 

 cases, not excepting the case of pure gases. 



XL VI. On the Theory of Pyro-electricity and, Piezo-electricity 

 of Crystals. By Lord Kelvin*. 



§ 1. npHE doctrine of bodily electro-polarization masked 

 JL by an induced superficial electrification, which I 

 gave thirty years ago in Nicholas Cyclopaedia] ', wanted a 

 physical explanation of the assumed molecular polarization 

 to render it a satisfying physical theory of pyro-electricity ; 

 and it was essentially defective, as has been remarked by 

 Rontgen J and by Yoigt §, in that it contained no suggestion 

 towards explaining the multiple electric polarities irregularly 

 produced by irregular changes of temperature in boracite, in 

 quartz, and in tourmaline itself ; which had perplexed many 

 naturalists and experimenters. A short but very important 

 paper by MM. Jacques and Pierre Curie in the Comptes 

 Rendus for Feb. 14, 1881, supplies that want in a manner 

 which suggests what seems to me the true matter-of-fact 

 electro-chemical theory of a crystalline molecule, and at the 

 same time makes easy the extension of my slight primitive 

 doctrine, to remedy its defect in respect to multipolarity, and 

 to render it available for explaining not only the old-known 

 pyro-electricity of crystals, but also the piezo-electricity dis- 

 covered by the brothers Curie || themselves. The element of 

 zinc and copper soldered together and surrounded only by 

 air, which they suggest, represents perfectly a true electro- 



* Communicated by the Author. 



t Reprinted in ' Collected Mathematical and Physical Papers ' (Sir 

 W. Thomson), vol. i. p. 315. 



X Wiedemann's Annalen, 1883, xviii. p. 213. 



§ " Allgemeine Theorie der Piez.o- und Pyro-electrischen Erschein- 

 ungen an Krystallen," p. 

 Abhand. Konig. Ges. Wiss. Gbttingen, 1890. 



|| Comptes Rendus, Aug. 2 and Aug. 10, 1880. 



