On the Humite of Monte Somma. 181 
These are each parts of distinct infinite series, and they indi- 
cate the character of the modification in the attracting power o: 
the molecules by which the secondary planes are produced. They . 
in fact, appear to contain the elements or data for deducing a gen- 
eral law at the basis of such modifications in the attraction, both 
M. Scacchi also points out a singu- 
lar example of hemihedrism observed in 
ctystals of the 2nd and 3d type, as well 
8S some compound crystals of the same 
pita airangement as in plagihedral 
quartz, and the spiral turn may be to 
the right or left. 
" * See this Journal, [2], iv, 373. 
