386 . Miscellanies. 



which is as common here in Pachuca, Zimapan, he. as In Corn- 

 wall ; and the quartz, which never occurs as rhombohedron, should 

 have been placed in the order of the regular hexagonal prisms for 

 the sake, I repeat, of the student per se. 



I did not allude (nor could I do so if in my right senses) to the 

 law of isomorphism of Mitscherlich, when I said that it is to be 

 inferred from his observations on biphosphate of soda, sulphur, 

 carbonate of lime in calcareous spar and in arragonite, and arsenious 

 acid and probably metallic arsenic also, that elementary and corri- 

 pound bodies are capable of assuming two distinct crystalline forms. 

 Professor Shepard may find it in Turner's Chemistry, page 688. 



The European sulphuret of manganese is the same species, or 

 genus, as the Mexican ; the 17 per cent of sulphur in the Europe- 

 an was a mistake ; Arfwedson has recently found 37 : consequent- 

 ly, the same species, or better to say, genus, crystallizes in two dif- 

 ferent systems, the hexahedron and the rhombohedron. 



I am sorry for my inapplicability of the 70th, 77th and 78th pa- 

 ges, for broken or imperfect crystals, (page 323.) The fact is, that 

 it is said in the 70th page, if we arrive at the knowledge of the 

 lateral faces of a prism, we possess independently of the cleavage, 

 means for determining the base, whether it be horizontal or oblique. 

 Now looking for these means, I found at the 76th page a remark 

 thus commencing : In consequence of the irregularity of crystals, 

 when it is said, page 78, let us suppose a crystal to be contained 

 within a series of vertical planes, and to be terminated, not by a 

 horizontal plane, but by a single oblique plane, it will belong to the 

 oblique rhombic prism, ^c. My crystal is now broken at the ex- 

 tremities, so that no terminal plane, either horizontal or oblique, is 

 to be discovered ; and it is clear that from one or two lateral faces 

 or vertical cleavages, no regular solid must result. What is to be 

 done ? The means, in spite of his copiousness, are no where to 

 be found in Professor Shepard's Mineralogy. 



I know pretty well, that the shorter the character, (page 324,) 

 the greater the facility and certainty it will afford in the distinc- 

 tion. Very true by essential characters ; but how many such are to 

 be found in the mineral kingdom ? Even the crystals cannot be 

 such ; since, according to Professor Breithaupt, the prisms of the 

 genus pyroxene vary no less than two degrees, and those of the ge- 

 nus amphibole nearly five. 



