230 Professor Necker on Mineralogy co?isidered 



§ 35. In what, then, does the relative vahie or importance of 

 characters consist ? The idea of the relative importance of charac- 

 ters is a complex one ; it is a compound of the consideration of 

 the dependence existing from more particular properties to more 

 general ones, and at the same time it takes into account the re- 

 lative number of less important characters subordinate, as well 

 as the relative number of species, genera, families to which the 

 property to be evaluated is common as a character ; in such a 

 way that a character will be reckoned the more important as it 

 is the more general, as it has under it a greater number of 

 subordinate characters, and also as it is the common tie of a 

 greater number of species, genera, and families. In comparing 

 different characters as to their relative importance, it must be 

 understood that the characters compared must belong to the 

 same nature of properties, so that properties relating to the 

 crystalline form should not be compared to chemical properties, 

 and vice versa. 



Thus in relation to the form of crystals, the shape and size of 

 the faces in a secondary crystal are characters of very little im- 

 portance, when compared to their number, their position in re- 

 gard to the axis, and their mutual incidences ; even these exter- 

 nal limitations of secondary crystals are of smaller value than 

 the determination of the fundamental form as given by the 

 cleavage ; lastly, the consideration of the system to which the 

 fundamental form belongs, is of a much higher degree of im- 

 portance than the specification of the fundamental form itself. 

 Under such a point of view, the arrangement of the crystallo- 

 graphical characters, according to their relative value, will be as 

 follows, beginning with the most important : 1^^ the system of 

 the fundamental form, and accordingly the optical properties as 

 to refracted and polarized light ; 2c?, the particular fundamen- 

 tal or primitive form, and with it the cleavage, and other con- 

 siderations, by means of which this form is ascertained ; Sd, 

 the derived or secondary form ; 4^A, and lastly, the accidental 

 variations in shape and size to which the limiting planes of se- 

 condary crystals are often liable. 



It is easy to see that, by analogous means, similar distinctions 

 may be introduced into the other sets or series of properties, 

 ^ither chemical or physical. 



