MINERALOGY. 547 



the molecular structure was isotropic, and the geometric form and oj^ti- 

 cal character .were iu harmony with each other. Under jireseut condi- 

 tions, however, the molecular structure is anisotropic, double refraction 

 has arisen from secondary causes, while the geometric form remains un- 

 3hauged — form and optical properties being then contradictory to each 

 other. This subject has been discussed by Klein [Jalirh. Min., 1884, i, 

 235), and he describes the best methods of observation in order to show 

 this change from the isotropic to the anisotroi)ic state and back again. 

 Mack has discussed at length ( Wied.Ann., xxi, 410) the pyro-electricity 

 of boracite, and shows the close relation which exists between the elec- 

 trical and the optical phenomena. 



It will be inferred from what has been said about boracite that a sim- 

 ilar explanation of the optical anomalies in other crystals may be ex- 

 pected to be found to hold true, although it is not to be supposed that 

 this is the only cause of the secondary double refraction shown by so 

 many minerals. Klein has extended his observations to a number of 

 other species, the most important of which is leucite. This species, 

 though long known to show more or less distinct double refraction, was 

 included in the isometric system until vom Eath (1873) showed that the 

 form could be interpreted as tetragonal ; since then most mineralogists 

 have accepted vom Rath's conclusion. It is now shown by Klein 

 {Nacliricliten K. Ges. Wiss. Gotiingen, May 3 and August 2, 1884) that at 

 an elevated temperature leucite also loses its double-refraction and be- 

 comes isotrojiic, and on cooling again becomes anisotropic, though the 

 structure was somewhat altered. From this observed fact, he naturally 

 concludes that at its formation the conditions were such that the min- 

 eral was isometric in form and its molecular structure correspondingly 

 isotropic. 



The memoir by Klein on leucite is a model of careful, accurate work. 

 In his introduction he describes the special form of microscope employed, 

 which had been constructed according to his plan, and which was in 

 many respects an improvement upon earlier forms (as thatof Bertrand) 

 made with the same object of allowing exact optical determinations. 

 The material under examination was ample, embracing crystals from 

 a variety of occurrences, and, of these, some 350 thin sections were 

 prepared. Without going too far into details, it is enough to state 

 the general conclusions, that in form and optical characters leucite cor- 

 responds to the orthorhombic system (as first urged on geometrical 

 grounds by Weisbach), but approximates closely to the tetragonal. The 

 structure is in general that of a complex twin, three fundamental indi- 

 viduals crossing each other, which may or may not be equally developed; 

 in some cases, one of these may predominate over the others. These in- 

 dividuals are twinned parallel to the planes which on the old view 

 would have been called a dodecahedral plane. A good deal of irregu- 

 larity is observed in the angles, both crystallograpliic and optical; it is 

 found, moreover, that the inclusions do not correspond in their distri- 



