II! A \<;\ \l. SYSTKM 



111 



Derived from the trigonal 

 pyramid of the third order 

 (here arc eight trigonal hemi- 

 pyramids, of which Fig. 213 

 represents the upper left 

 minus hemipyramid, and the 

 poles in Fig. 212 are of the 

 upper riffht plus form. 



II. Trigonal hemipyramids 

 of the second order, 

 2a: 2a: a: mc\ 



Fia. 213. The Upper Left Negative Trigo- 

 nal Hcmipyramid of the Third Order. 



(Utahl), (hhahl), (ahfiU), ( 2 hhhl). 



There are four trigonal hemipyramids of the second order, de- 

 rived from the trigonal pyramid of the second order, all of which 

 may occur independently. Fig. 214 is the upper minus trigonal 

 bemipyromid of the second order. 



The forms possible to combine in this type are as follows : 

 Plus and minus right and left upper and lower hemipyramids of 

 the third order, (hkil), (khil), (ihkl), (ikhl), (hkil) , (khfl) , (ihkl), (ikM) . 



Fia. 214. The Upper Negative Trigonal Hemi- FIG. 215. Combination of 

 pyramid of the Second Order. Forms showing the Polar 



Development of Crystals of 

 Sodium Periodate. 



Plus and minus upper and lower trigonal hemipyramids of the 

 second order, (hh2hl) ; (2hhM), (hh2~hT), (2hhhT). 



Plus and minus upper and lower trigonal hemipyramids of the 

 first order, (hohl), (ohhl), (hohi), (ohhl). 



Plus and minus right and left trigonal prisms of the third order, 

 (hkio). (ihko), (khio), (ikho). 



Plus and minus trigonal prisms of the second order, (hoho) , (ohho) . 



Plus and minus trigonal prisms of the first order, (hh2ho), (2hhho). 



Upper and lower base, (0001), (OOOl). 



