. 155. OF COMBINATIONS. 203 



edges of combination are horizontal. This prism is rect- 

 angular if the base of the fundamental form is a rhomb, 

 but its transverse section will differ more or less from a 

 rectangular figure, if this base be a rhomboid. 



Examples of tetarto-prismatic combinations occur in te- 

 tarto-prismatic Vitriol-salt, in several species of the genus 

 Feld-spar, in prismatic Axinite, and other species. 



. 155. DEVELOPEMENT OF PRISMATIC COMBINA- 

 TIONS. 



The following developement will fully explain 

 the application of what has been stated above in 

 respect to binary combinations. 



Fig. 72. represents a prismatic combination, indetermi- 

 nately designated by 



Among the simple forms, constituting this compound 

 one, two are immediately determined, P + co (/) and 

 Pr -f co (/*). 



If we suppose n 1 = 0, P + n 1 (e) becomes = P, that is 

 to say, the fundamental form. On account of the parallel 

 edges of combination between P and Pr + n m (c), n 111 is 



In the indeterminate designation both the signs 

 (. 192. 195.) are made use of before the forms have 

 been determined, till it appears from the developement ac- 

 cording to the reasons given above, which of the two is to 

 be retained. 



