628 ADDITIONS. 



Professor Dana, in his System of Mineralogy, New Haven (U.S.), 

 1837, follows Naumann for the most part, both in crystallography and 

 in mineral classification. In the latter part of the subject, he has made 

 the attempt, which in all cases is a source of confusion and' of failure, 

 to introduce a whole system of new names of the members of his 

 classification. 



The geometry of crystallography has been investigated in a very 

 original manner by M. Bravais, in papers published in the Journal of 

 the Ecole Polytechnique, entitled Memoires sur les Systemes formes 

 par des Points. 1850. Etudes Crystalloyrapliiques. 1851. 



Hermann Kopp (Eirileitung in die Krystallographie, Braunschweig, 

 1849) has given the description and measurement of the angles of a 

 large number of laboratory crystals. 



Rammelsberg (Krystallographische Chemie, Berlin, 1855) has col- 

 lected an account of the systems, simple forms and angles of all the 

 laboratory crystals of which he could obtain descriptions. 



Schabus of Vienna (Bestimmung der Krystallgestalten in Chemi- 

 schen Laboratories, erzeugten Producte, Wien, 1855 ; a successful Prize 

 Essay) has given a description, accompanied by measurements, of 90 

 crystalline species from his own observations. 



To these attempts made in other countries to simplify and improve 

 crystallography, I may add a remarkable Essay very recently made 

 here by Mr. Brooke, and suggested to him by his exact and familiar 

 knowledge of Mineralogy. It is to this effect. All the crystalline 

 forms of any given mineral species are derived from the primitive form 

 of that species ; and the degree of symmetry, and the parameters, of 

 this form determine the angles of all derivative forms. But how is 

 this primitive form selected and its parameters determined ? The 

 selection of the kind of the primitive form depends upon the degree of 

 symmetry which appears in all the derivative forms; according to 

 which they belong to the rhombohedral, prismatic, square pyramidal, 

 or some other system : and this determination is commonly clear. 

 But the parameters, or the angles, of the primitive form, are commonly 

 determined by the cleavage of the mineral. Is this a sufficient and 

 necessary ground of such determination ? May not a simplification be 

 effected, in some cases, by taking some other parameters ? by taking a 

 primitive form which belongs to the proper system, but which has some 

 other angles than those given by cleavage ? Mr. Brooke has tried 

 whether, for instance, crystals of the rhombohedral system may not be 

 referred with advantage to primitive rhombohedrons which have, in all 



