LORD KELVIN. AEPIXUS ATOMIZED. 



855 



around the solid. If at tlie altérée! température or under the supposed 

 stress the solid is again dipped in water and dried, the external electric 

 force will be again annnlled. Thus is explained the pyro-electricity of 

 tonrnaline discovered by àepinus. 



§ 34. But a merely di-polar electric crystal with its single axis pré- 

 sents to us only a small, and the very simplest, part of the whole subject 

 of electro-crystallography. In boracite, a crystal of the cubic class, 

 Hauy found in the four diagonals of the cube, or the perpendiculars to 

 the four faces of the regular octahedron, four di-polar axes: the crystal 

 on being irregularly heated or cooled showed as it were opposite electri- 

 cities on the surfaces in the neighbourhood of opposite pairs of corners of 

 the cube, or around the centres of the opposite pairs of triangular faces of 

 the octahedron. His discoveries allow us to conclude that in gênerai 

 the electric aeolotropy of crystals is octopolar with four axes, not merely 

 dipolar as in the old-known electricity of the tourmaline. The intensi- 

 ties of the electric virtue are generally différent for the four axes, and 

 the directions of the axes are in gênerai unsymmetrically oriented for 

 crystals of the unsymmetrical classes. For crystals of the optically 

 uniaxal class, one of the electro-polar axes must generally coincide 

 with the optic axis, and the other three may be perpendicular to it. The 

 intensities of the electro-polar virtue are essentially equal for thèse 

 three axes: it may be null for each of them: it may be null or of any 

 value for the so-called optic axis. Haûy found geometrical différences 

 in respect to crystalline facets at the two ends of a tourmaline; and be- 

 tween the opposite corners of cubes, as leucite, which possess electro- 

 polarity. There are no such différences between the two ends of a quartz 

 crystal (hexagonal prism with hexagonal pyramids at the two ends) but 

 there are structural différences (visible or invisible) between the op- 

 posite eclges of the hexagonal prism. The electro-polar virtue is null 

 for the axis of the prism, and is proved to exist between the opposite 

 edges by the beautiful piezo-electric discovery of the brothers Curie, 

 according to which a thin flat bar, eut with its faces and its length 

 perpendicular to two parallel faces of the hexagonal prism and its 

 breadth parallel to the edges of the prism, shows opposite electricities 

 on its two faces, when stretched by forces pulling its ends. This proves 

 the three electropolar axes to bisect the 120° angles between the consé- 

 cutive plane faces of the prism. 



