TKAN3ACTI0NS OF SECTION C. 639 



The controversy, however, passed to a fresh stage when it was discovered that 

 boracite becomes optically isotropic when sufficiently heated, and resumes an 

 optically composite character on cooling'. Mallard showed that the temperature at 

 which the change takes place is a definite one, 265° C.,and that a definite amount 

 of heat is absorbed or given out during the change of condition. 



It is now agreed that boracite is really dimorphous ; that above 265° it is cubic 

 in symmetry, below 265° orthorhombic : the only remaining point of controversy 

 as regards boracite seems to be whether the external form owes its cubic symmetry 

 to the crystallisation having taken place at a temperature higher than 265°, and 

 therefore when the structure itself was truly cubic, or at a temperature below 

 265°, in which case the cubic character of the form would be ascribed to the fact 

 that the orthorhombic constituent particles are so nearly cubic in their dimensions 

 that at any temperature they may by variety of orientation combine to form a 

 structure having practically cubic symmetry, and naturally limiting itself by faces 

 corresponding to such a symmetry. 



In exactly the same way leucite and tridymite become respectively optically 

 isotropic and uniaxal when sufficiently heated, and the optical characters then 

 correspond exactly to the symmetry of the external form. 



Three years ago Dr. Brauns pre-pared a most useful summary of the ninety-four 

 memoirs which had up to that time been contributed relative to the much-dis- 

 cussed subject of the optical anomalies of crystals, and added many new experi- 

 mental results which had been obtained by himself He concludes that the 

 original view of Mallard — namely, that an optically anomalous structure consists 

 merely of differently orientated particles of the same kind and of symmetry 

 approximating to a higher type — is only applicable to a very limited number of 

 crystals, such as those of prehnite ; that dimorphism is the true cause in others, 

 boracite being an example ; that in the remaining minerals the cause is strain, 

 which in some of thera is due to foreign enclosures, as in the case of the diamond, 

 and in others is due to a molecular action between isomorphous substances, as in 

 the mixed alums and the garnets. 



Planes of Gliding. — One of the most startling of crystallographic discoveries 

 was one made by Reusch, who found that if a crystal of calcite is compressed in a 

 certain way each particle springs into a new but definite position, exactly as if the 

 crystal had undergone a simple shear and the particles at the same time had each 

 described a semi-somersault: a simpler method of produring the same result was 

 discovered afterwards by Baumhauer. If only part of the calcite crystal is sheared, 

 the two parts of the structure itself are related to each other in the same way as 

 the two parts of a twin growth ; but in general the external form is different from 

 that of a twin, since after the shearing of the material few of the faces retain their 

 former crystallographic signification. The property has since been shown by Bauer, 

 Liebisch, and more especially Miigge, to be a very general one ; and doubtless the 

 so-called twin lamellae met with in rock-constituents have in many cases resulted 

 from pressure during earth-movements long subsequent to the epoch of formation 

 of the crystals. Similar lamellae have been produced artificially in anhydrite and 

 some kinds of felspar by exposure of the crystals to a high temperature. 



Piezo-electricity. — The most remarkable addition to our knowledsre of the 

 relation of minerals and electricity has been the recent discovery of the electrifica- 

 tion produced by strain (pifzo-electricity). It has been shown by J. and P. Curie 

 that if a quartz-plate, with faces cut parallel to the axis and silvered to make them 

 conductive, be strained in a certain direction, the two faces either become oppositely 

 electrified or show no signs of electrification at all, according as the faces of the 

 plate are cut to be perpendicular to the prism-faces, or to p«ss through the prism- 

 edges. Lord Kelvin says that this result is explicable by electric eolotropy of the 

 molecule and by nothing else, a character which he had suggested for the molecule 

 thirty-four years ago : experiments confirmatory of this hypothesis of the permanent 

 electrification of the molecule were made some time ago by Riecke. 



Fyro-electricity. — The development of opposite electricities at different parts of 

 a crystal during changirg temperature (pyro-electricity) has long bt^en known in 

 the case of tourmaline. We owe to Hankel a long series of investigations of this 



