88 THE MICROSCOPIST. 



and inclined 60 to each other, and a principal axis at 

 right angles to the plane of the others. Examples, quartz, 

 beryl, and calc-spar. 



4. Rhombic or Trimetric System (Fig. 42). Three rec- 

 tangular axes, all of different lengths. Examples, sulphate 

 of potassium, nitrate of potassium, sulphate of barium, 

 and sulphate of magnesium. 



5. Oblique Prismatic or Monoclinic (Fig. 43). Two axes 

 obliquely inclined, and a third at right angles to the plane 

 of these two ; all three being unequal. Examples, ferrous 

 sulphate, sugar, gypsum, and tartaric acid. 



6. Didinic System. Two axes at right angles, and a 

 third oblique to the plane of these ; the primary form 

 being a symmetrical eight sided pyramid. 



7. Doubly Oblique Prismatic or Tridinic (Fig. 44). Three 

 axes all inclined obliquely and of equal length. Example, 

 sulphate of copper. 



Crystalline structure being inherent in the nature of 

 the mineral, becomes perceptible by the manner of divi- 

 sion. A slight blow on a piece of calc-spar will separate 

 it into small rhombohedrons or parallelopipeds, or produce 

 internal fissures along the planes of cleavage, which will 

 suffice to determine their angles. 



Crystals are often found in groups, with various modes 

 of arrangement. Cubes are sometimes aggregated so as 

 to form octahedra, and prismatic crystals are often united 

 together at one extremity. But the most singular groups 

 are those called hemi tropes, because they resemble a crys- 

 tal cut in two, with one part turned half round and re- 

 united to the other. 



In all the numerous forms, however, we find in the same 

 species the same angles or inclination of planes, although 

 the unequal size of the faces may lead to great apparent 

 irregularity, as in distorted crystals of quartz, where one 

 face of the pyramid is enlarged at the expense of the rest. 



