274 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



bearing on the crystallographic relations of a large group 

 of minerals. He describes the antimony iodide as obtained 

 by him in three distinct modifications, belonging respective- 

 ly to the hexagonal, orthorhombic, and monoclinic systems, 

 and differing in color and melting-point. lie found, more- 

 over, that both the last two varieties, under proper condi- 

 tions as to temperature, could be changed into the hexago- 

 nal variety, becoming red instead of yellow, and optically uni- 

 axial instead of biaxial. Professor Cooke explains the change 

 by a kind of molecular twinning or rearrangement of the mol- 

 ecules. The change is analogous to that which, as explained 

 by the same author, and also independently by Keusch, is 

 produced when thin plates of muscovite are combined in 

 symmetrical positions, so as to produce uniaxial optical ef- 

 fects. The difference in the antimony compound is, how- 

 ever, truly molecular instead of laminar, and the author sug- 

 gests that it may be a similar relation which determines the 

 difference between calcite and aragonite. The number of 

 minerals which, crystallizing in the orthorhombic (or mono- 

 clinic) system, and having a prismatic angle of nearly 120, 

 show in their twins a pseudo-hexagonal symmetry is quite 

 large ; to them the facts brought out by Professor Cooke 

 have an immediate bearing. 



The improved methods of research now employed, includ- 

 ing the measuring of the angles of crystals with extreme 

 accuracy, and, still more important, the careful examination 

 of their optical properties, have led in many cases to a 

 change in the system to which the crystals of a species have 

 been referred. The tendency is thus very strong to throw 

 the crystals of the more simple systems into those which 

 are more complex and have a lower grade of symmetry. 

 This is illustrated by the change accepted some years since 

 in the case of leucite, formerly regarded as a typical isomet- 

 ric species, now referred to the tetragonal system. An ex- 

 tended article upon the subject, in part based upon experi- 

 ments and in part simply speculative, has been published by 

 Mallard. lie takes up in succession a large number of spe- 

 cies which have been long known to present anomalous opti- 

 cal characters, lie rejects the explanations offered by oth- 

 ers, as, for instance, the theory of lamellar polarization ad- 

 vanced by ]jiot, and offers another of his own, which, howev- 



