EXTERNAL CHARACTERS OF MINERALS. 101 



correctly measuring the angles of crystals, and, as in the 

 instance of augite and hornblende, for distinguishing several 

 which present considerable similarity to each other. The 

 use of the instrument is not particularly dinicult, and the 

 student is referred to Phillips, fourth edition, Introduction, 

 p. xxxii, for directions, the observance of which will speedily 

 enable him to attain the management of it with ease and 

 success. 



In studying the characters of minerals according to the 

 arrangement of Phillips, the student should commence by 

 a careful and attentive perusal of the admirable Introduction 

 from page xiii to page Ixviii. The various external charac- 

 ters detailed in page xviii should be the object of especial 

 study, more particularly those enumerated in the first 

 column in the following list ; and the chapter on Structure, 

 above all, cannot be too firmly impressed on the mind. 



They are as follow : 



External form 



Structure 



Fracture 



Frangibility 



Hardness 



Streak 



Magnetism 



Electricity 



Specific gravity 



Transparency 



Lustre 



Colour 



Flexibility 



Double refraction 



Touch, taste, and odour 



Powder 



Adhesion to the tongue 



Phosphorescence. 



CLEAVAGE. It may be necessary to add a few remarks 

 on this subject, which is not particularly dwelt .on in the 

 work of Phillips. It means the indications of the faces of 

 the primary crystal, or at least of some of them. There are 

 several minerals which possess cleavage so perfect, that they 

 at once exhibit the crystalline faces, by splitting into frag- 

 ments, having each the crystalline shape of the mineral ; as 

 calcareous spar, which breaks into rhomboids, and galena, 

 which divides into cubes. Other minerals require more care ; 

 sometimes the natural joint may be seen by turning the 

 crystal round in a strong light ; and by the application of a 

 chisel, or a small knife, it may be cleaved in those directions ; 

 or it may be split by means of a pair of small cutting pincers, 

 whose edges are parallel. A small, short chisel, fixed, with 

 its edge outward, in a block of wood, is a convenient 



