OF ARTS AND SCIENCES. 63 



by which we have obtained identical phenomena in our artiticial plates 

 of mica, and that the two ordei's of" crystals are aggregates of compound 

 molecules, whose parts are twinned together in the one case in right- 

 handed, and in the other in left-handed spirals, and, lastly, that the 

 simple molecule, if developed normally, would produce a biaxial 

 structure* This theorj' is most markedly in harmony with the chemi- 

 cal relations of silica. The compound SiO^ is the only one of the 

 tetrad oxides which crystallizes in the hexagonal system ; and ever 

 since, by the study of the organic compounds of silicon, the quadriva- 

 lent character of the element has been made evident, this fact has been 

 a striking anomaly in our chemical classification. Assume, however, 

 that the molecule SiO.^ would develop normally into a rhombic struct- 

 ure, and that the hexagonal form of quartz is solely a result of molec- 

 ular twinning, and the anomaly disappears. The molecule SIO., may 

 be approximately of the same form as the molecule TiO.^, in Brookite; 

 but, having the exact dimensions and polar conditions which favor the 

 mode of molecular twinning, described above and represented by Fig. 7, 

 it may always develop into hexagonal shapes. 



Are, then, all hexagonal forms thus closely related to the rhombic 

 systems of crystals ? And do all molecules of the dimensions and 

 polar conditions illustrated by the figures of this article — that is, 

 those which correspond to the rhomb of 60° and 120*^ — usually 

 develop into hexagonal forms ? May not the whole difference between 

 an hexagonal and a rhombic form arise from a slight difference of dimen- 

 sions, which determines a molecular macling in the one case, and a 

 normal development of the single molecules in the other ? These 

 questions point out most interesting lines of investigation, and will 

 recall to the mineralogist a number of facts bearing upon the subject. 

 Allow me to refer to two of the most striking and most obvious. 



On the crystals of chrysoberyl, the rhombic angle is 119° 46'; 

 and every mineralogist is familiar with the hexagonal macling, similar 



* Since the above was in type, we have received Am. Jour. Sci., IV., Feb- 

 ruary, 1874, containing a description of the rliombic silica wliich Professor Mas- 

 kelyne, of the Britisli Museum, has discovered in the meteorite of Breitenbach. 

 This new species of silica, which Professor Maskelyne calls Asmanite, has the 

 form of a right rhombic prism, with an angle of 120° 20', and the crystals are 

 optically biaxial ; but while the specific gravity of quartz is 2.6, that of Asmanite 

 is said to be 2.245. It is perhaps to be expected that such a molecular macling 

 as we have described would determine an increase of density, since thereby three 

 molecules coalesce to form one; or it is possible that the remarks made in regard 

 to calcite beyond apply also to quartz ; but still the marked difference remains 

 to be explained. 



