56 On the Homceomorphism of Mineral /Species 



46', whence another macrodome 21 = 70° 50', or very nearly 

 the angle of the brachydome of Valentinite. It gives for i2, 

 the angle 100° 58', as mentioned in the table. The occurrence 

 of these Monometric angles has, beyond doubt, a profound sig- 

 nificance. We hereby perceive in what respect section II. is 

 related to sections I. and III. The oscillations from the typical 

 angles of the group amount to about 5°. 



Other species in Table II. require special remarks. 



In Sulphur, the unit macrodome of authors has the summit 

 angle 46° 18'. But taking fZ as the unit dome, the angle is 

 near that of Barytes and the other sulphates, as in the table ; 

 yi in Sulphur is near hi in Barytes. The homceomorphism of 

 Sulphur and the sulphates (RO, SO 3 ) is hence evident. 



Orpiment (sulphuret of arsenic) differs from Sulphur in per- 

 taining to section II. The Sulphur and other arsenic compounds 

 present a like amount of difference ; and, further, they show 

 that the fundamental vertical prism of Orpiment is that of 100° 

 40', instead of that of 117° 49', adopted by some authors. The 

 difference in the unit domes of Sulphur and Orpiment is about 

 7£ degrees, and the difference in the sulphurets and arseniurets 

 or arsenio-sulphurets is nearly as large, or 5 to 6 degrees. 



The arseniuret of iron (Leucopyrite or Lolingite) has been 

 hitherto described as having a vertical prism of 122°, and the 

 arsenio-sulphuret (mispickel) an angle of 112°, and the sul- 

 phuret (marcasite) of 106°. But the writer has been informed 

 by R. P. Greg, Jr., that he has measured crystals of the arse- 

 niuret, and found the angle the same as for the arsenio-sulphuret. 

 The difference of 6 degrees is, therefore, the full difference due 

 to the arsenic; and where arsenic is present with sulphur in 

 these compounds, the sulphur is wholly overpowered by the 

 arsenic ; just as in the sulphato-carbonates, the sulphuric acid 

 dominates completely over the carbonic acid, the angle agree- 

 ing closely with that of the sulphates (anglesite), instead of 

 being half way between those of the carbonates and sulphates. 



Dimorphine, a sulphuret of arsenic of undetermined compo- 



