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MEASUREMENT OF CAPACITY. DIELECTRICS. 



on the other, and for a given point is proportional to the distance 

 from this point to the median section. The quantity of electricity 

 produced on each terminal is proportional to the extent of this 

 surface, and is sensibly proportional to the difference of the tem- 

 peratures. 



We may state, again, that the apparent polarization is pro- 

 portional to the expansion for unit length between the temperatures 

 in question. 



1076. Pyroelectric polarization is connected with hemihedry of 

 oblique faces, as Haiiy* long ago remarked. 



In tourmaline which crystallizes in rhombohedra (Fig. 229), the 

 faces which terminate the cylindrical rods are not distributed in the 

 same way at the two poles. On the analogous pole, the faces P of 



the primitive rhombohedron rest on the three lateral faces of the 

 triangular prism e 2 , and the hemihedral elements are represented by 

 the facets I 1 and e l of two distinct rhombohedra. 



Hydrated silicate of zinc, or calamine, which crystallizes as a right 

 rhombohedral prism, is also pyroelectric, and the axis of polarization 

 is parallel to that axis of the crystal which has the characteristic of 

 hemihedry. In Fig. 230, when this axis is vertical, the analogous 

 pole is terminated by a horizontal truncature P, and the antilogous 

 pole by the summit of a octahedron. 



Pyroelectrical phenomena have been observed in a great number 

 of mineral substances belonging to various crystalline systems 

 boratite, scolezite, prehnite, topaz, etc.; and, among artificial crystals, 

 struvite, the tartaric acids, tartrates, etc. 



* HAUY. Ann. de Chim. et de Phys. [i], Vol. ix., p. 59. 1800. 



