Prof. Naumann on Mineralogical Classification. 35 



not unfrequently crystals reflect different kinds of lustre from 

 their different faces, and from the fact, that certain modifi- 

 cations of lustre arise rather from the state of aggregation 

 than from the mass itself. It is self-evident, that the inten- 

 sity of the lustre, which is dependent on the nature of the 

 surface, is still less deserving of general attention. 



Pellucidity, w^hich is susceptible of so many gradations 

 from transparency to opacity, must be taken into considera- 

 tion, not according to its degree, but with reference to its 

 being present, because the opposite of pellucidity, viz., opa- 

 city, is a character of the metallic habitus. Hence we must 

 avoid, as much as possible, intermixing pellucid and opaque 

 minerals ; and exceptions to this rule are only to be admit- 

 ted in special cases.* 



Hardness and specific gravity, two properties which per- 

 foi-m so important a part in the determination of species, 

 also deserve proper attention in classification. As, however, 

 they admit only of quantitative and not of qualitative distinc- 

 tions ; as in respect to them, the similarity of two mineral 

 species can only be sought for in the presence of gradations 

 approaching each other very nearly, or at least not at all 

 remotely ; and as an absolute determination and comparison 

 of such gradations is not afforded to us, more especially in 

 the case of the hardness ; we can only state, generally, that, 

 in each group, thei'e niust not be too strikingly different de- 

 grees of hardness and too great a variation in the specific 

 gravity, although even here, and, more particularly, in re- 

 gard to the hardness, there may be special exceptions, when 

 there is agreement in more important properties. Much 

 greater attention is due to specific gravity than to hardness, 

 because the uncertainties connected with the former, are 

 fewer in number and smaller in amount, and because its 



* Connected with this point, the occurrence of what is termed the 

 splintery fracture is a character deserving of attention, because it al- 

 ways indicates to us the presence of pellucidity, even when the latter is 

 only perceptible as a feeble translucenc}' in the thinnest splinters. Opaque 

 minerals never admit of our recognisiiKj the splintery fracture, even 

 though it should actually be present. 



