of the Sulphato-tri-Carbonate of Lead. 



mined by the parallelism between o, e and o, and by that be- 

 tween h, o and b. From the first of these data, it appears, that 

 the terminal edge of the pyramid o, contiguous to the long 

 diagonal of the fundamental form, is inclined to the axis under 

 the same quantity as the analogous edge of P+l, and that, 

 consequently, a' : b' will be in the ratio of 2 a : b. But, from the 

 second datum, it is evident, that the terminal edge of the same 

 pyramid, contiguous to the short diagonal of the fundamental form, 



u 



will be situated like the analogous line of h, which is ^ "^ ; - ; 

 and this makes the ratio of a' : c' a : 4c =. 2 a : 8c. The ratio be- 

 tween the three lines a' : b' : c' ~ ~ : b : 8c, in the derived pyra- 

 mid, is expressed by the crystallographic sign (p 2) 8 . There 

 are some slight indications of faces between f and o, and between 

 o and d ; but they are too imperfect to allow of any accurate de- 

 termination. The rest of the faces which appear in the combi- 

 nation, are easily referred to those forms to which they belong, 

 because they possess on the opposite side of the axis a situation 

 perfectly analogous to that of the forms developed in the pre- 

 ceding observations. Thus, g' is analogous to g, and therefore 



o 



= ^-^5 h' analogous to h ^ '~ 2 ^ ; k' analogous to k 

 (Prl) 3 . O f analogous to o (P ^) 8 . The designation 

 of the whole form is, 



Pr 1 Pr 

 P-co. 2_' -. 



a i f 



u 

 I p O\ 8 



l r - *> p r o pZ i 



* i ~ ~ *. Jrr i. 



o I m 

 (Pr 1\ 3 P fp 2,\* 



- sj - r - *3p' p+- (+) 4 - pr+. 



V P o' c d b 



Vf2 



