494 GEOLOGY AND MINERALOGY. 



makes, consequently, the atomic constitution of Silica = SiO^. Silica and 

 Stannic Acid in their mineral forms, it is -well known, crystallize in different 

 systems, but Gr. Rose seeks to establish a crystallograpliic connexion between thetn 

 by the intervention of Zircon, a mineral long known to be identical in form with 

 cnssiterite, SnO^. He makes the formula of Zireonia = ZrO^ + ; and that 

 of Zircon ZrO^ + SiO-; viewing the latter as an isomorphous combination of 

 Silica and Zireonia; and, regarding it by inference consequently, as a proof 

 of the dimorphism of silica ; or, in other words, as an example of silica 

 crystallising in the dimetric system. This reasoning, however, appears to be 

 somewhat forced. Zircon crystallises in the dimetric system, in all probability, 

 under the crystallographie influence (so to say) of the Zireonia which forms 

 its more essential or predominating constituent. That one of the components 

 in crystal compounds frequently exerts an influence of this kind, is an assump- 

 tion now generally received. And again, by legitimately following out the 

 argument of Gustav Rose, we ought equally to consider silica a sesquioxide, seeing 

 that it occabionally replaces alumina, not only in such compounds as Augite and 

 Hornblende, but also in the Staurolites. With regard to the latter. Professor Rose 

 has himself stated in another place (Kri/siallo-chemi.-^che Mineralsystem, p. 77,) 

 that the only way of explaining their variable composition, is to assume the iso- 

 morphism of silica and alumina. And if, as he states further in objection to this, 

 no examples of these bodies have yet been found in identical forms, the same 

 argument still holds good with respect to silica and stannic acid on the one hand, 

 and silica and zireonia on the other. The modern progress of mineralogy tends 

 to bring out very prominently two facts : — First, that we should make a careful 

 distinction between simply isomorphous and truly vicarious bodies; and secondly 

 that the assumed atomic constitutions of mineral compounds must be allowed no 

 preponderating place in questions belonging to the philosophy of that science. 



Crystal Form of Oxide of Copper, — Dr. Jenzseh has described some crystals 

 of Black Oxide of Copper from some smelting works near Freiberg. The}'- 

 were obtained from furnaces in which argentiferous copper ores are roasted with 

 common salt, and they are considered due to the alteration of volatilized chloride 

 of copper. They belong to the Trimotric system. Simple crystals are rare, twins 

 and compound groups predominating. The twin-face is one of the planes of the 

 predominating rhombic pi'ism, and the re-entering angle equals 160° 42' SO". The 

 prism-angle itself (F: V = I:I, Dana = Poo : Pco .J^aumann) =99° 38' 45", and 

 80° 21' 15", the latter being, of course, just one-half the amount of the re-entering 

 angle. The crystals are mostly tabular, from the abnormal development of two 

 of the opposite V faces or vertical planes of the rhombic prism. Oxide of copper 

 (Cu O.) is thus shewn to be dimorphous, if not trimorphous. Becquerel obtained 

 it in monometric tetrahedrons by the fusion of the finely commiauted oxide with 

 potash ; and the Tenorite from the lava of Vesuvius is looked upon as Hexagonal 

 by some observers. Very probably, however, as pointed out by Dr. Jenzseh, the 

 six-sided scales in which the latter chiefly occurs, are really Trimetric. Dana's 

 Melaconite, another form of Cu from Lake Superior, sometimes occurs in small 

 cubo-octahedrons, it will be lemembered ; but those have been regarded (although 

 doubtfully) as pseudoraorphs derived from the red oxide Cu^ 0. The crystals 



