58 (hi the Homceomorphism of Mineral Species 



Scacchi), and f 2 (not observed) = 71° 56'. They approach most 

 nearly the unit domes of Anhydrite. 



Tantalite (FeO, TaO s ) has very nearly the dimensions of 

 Barytes (BaO, SO 3 ), as seen in the table ; and the fact is im- 

 portant, as it sustains the homceomorphism of tantalic and sul- 

 phuric acids. 



Broohite was first observed to be homceomorphous with 

 Columbite by Hermann. It differs by four degrees in its domes 

 from that species, and has its vertical axis about one twelfth 

 longer. 



In Columbite, it is of importance to note, that the face of 

 twin composition is a plane of the brachydome 23, in which 

 the basal angle is about 120° ; and in Wolfram, it is the brachy- 

 dome fi, in which the summit angle is about 120°. 



Leadhillite has been shown by the writer to have close rela- 

 tions in angle to Anglesite, in the American Journal of Science^ 

 vol. xvii., p. 210. Its dimensions, as given in the table, exhibit 

 still further this similarity of form. We reserve remarks on 

 the forms of Leadhillite for another occasion. 



Mascagnine diverges widely from the other sulphates in its 

 vertical prism, and therefore also in its brachydome, while it 

 agrees with them nearly in its macrodome. 



We add a word on the unit-octahedrons of the species of 

 Table II. The following are the angles for a species or two 

 in sections I., III., and IV. : 



It will be observed that there is an approximation to the 

 angle of a regular octahedron only in one of the pyramidal 

 angles of section I., and in the basal angle of section III. 



